internal/kibana: fix datasourceConnectorRead error

[irregulator]

Make sure that datasourceConnectorRead() returns the appropriate error when no connectors are found.

[irregulator]

Currently, the provider panics when no Kibana connectors are found:

➜  test_elasticstack_provider cat main.tf
data "elasticstack_kibana_action_connector" "my-connector" {
  name              = "doesnotexist"
  connector_type_id = ".pagerduty"
}

➜  test_elasticstack_provider terraform plan                                              
data.elasticstack_kibana_action_connector.my-connector: Reading...

Planning failed. Terraform encountered an error while generating this plan.

╷
│ Error: Request cancelled
│ 
│ The plugin6.(*GRPCProvider).ReadDataSource request was cancelled.
╵

Stack trace from the terraform-provider-elasticstack_v0.11.6 plugin:

panic: runtime error: index out of range [0] with length 0

goroutine 81 [running]:
github.com/elastic/terraform-provider-elasticstack/internal/kibana.datasourceConnectorRead({0x1752640, 0xc0003ae460}, 0xc000518e00, {0x14d2e20?, 0xc00098e000?})
        github.com/elastic/terraform-provider-elasticstack/internal/kibana/connector_data_source.go:86 +0x4c5
github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema.(*Resource).read(0xc0003610a0, {0x1752598, 0xc0006d95f0}, 0xc000518e00, {0x14d2e20, 0xc00098e000})
        github.com/hashicorp/terraform-plugin-sdk/v2@v2.34.0/helper/schema/resource.go:823 +0x119
github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema.(*Resource).ReadDataApply(0xc0003610a0, {0x1752598, 0xc0006d95f0}, 0xc000518d00, {0x14d2e20, 0xc00098e000})
        github.com/hashicorp/terraform-plugin-sdk/v2@v2.34.0/helper/schema/resource.go:1043 +0x13a
github.com/hashicorp/terraform-plugin-sdk/v2/helper/schema.(*GRPCProviderServer).ReadDataSource(0xc0000119c8, {0x1752598?, 0xc0006d94a0?}, 0xc0006d9560)
        github.com/hashicorp/terraform-plugin-sdk/v2@v2.34.0/helper/schema/grpc_provider.go:1434 +0x6b1
github.com/hashicorp/terraform-plugin-mux/tf5to6server.v5tov6Server.ReadDataSource({{0x175c9e8?, 0xc0000119c8?}}, {0x1752598?, 0xc0006d94a0?}, 0xc0006d9170?)
        github.com/hashicorp/terraform-plugin-mux@v0.16.0/tf5to6server/tf5to6server.go:142 +0x282
github.com/hashicorp/terraform-plugin-mux/tf6muxserver.(*muxServer).ReadDataSource(0xc0003aed20, {0x1752598?, 0xc0006d91d0?}, 0xc0006d9170)
        github.com/hashicorp/terraform-plugin-mux@v0.16.0/tf6muxserver/mux_server_ReadDataSource.go:36 +0x193
github.com/hashicorp/terraform-plugin-go/tfprotov6/tf6server.(*server).ReadDataSource(0xc0003a8960, {0x1752598?, 0xc0006d83c0?}, 0xc00032cf00)
        github.com/hashicorp/terraform-plugin-go@v0.23.0/tfprotov6/tf6server/server.go:688 +0x27d
github.com/hashicorp/terraform-plugin-go/tfprotov6/internal/tfplugin6._Provider_ReadDataSource_Handler({0x14fbba0, 0xc0003a8960}, {0x1752598, 0xc0006d83c0}, 0xc000518800, 0x0)
        github.com/hashicorp/terraform-plugin-go@v0.23.0/tfprotov6/internal/tfplugin6/tfplugin6_grpc.pb.go:572 +0x1a6
google.golang.org/grpc.(*Server).processUnaryRPC(0xc0001cd200, {0x1752598, 0xc0006d8330}, {0x1758f40, 0xc0001f5080}, 0xc0005bfb00, 0xc000472120, 0x225e7a0, 0x0)
        google.golang.org/grpc@v1.65.0/server.go:1379 +0xdf8
google.golang.org/grpc.(*Server).handleStream(0xc0001cd200, {0x1758f40, 0xc0001f5080}, 0xc0005bfb00)
        google.golang.org/grpc@v1.65.0/server.go:1790 +0xe8b
google.golang.org/grpc.(*Server).serveStreams.func2.1()
        google.golang.org/grpc@v1.65.0/server.go:1029 +0x8b
created by google.golang.org/grpc.(*Server).serveStreams.func2 in goroutine 53
        google.golang.org/grpc@v1.65.0/server.go:1040 +0x125

Error: The terraform-provider-elasticstack_v0.11.6 plugin crashed!

This is always indicative of a bug within the plugin. It would be immensely
helpful if you could report the crash with the plugin's maintainers so that it
can be fixed. The output above should help diagnose the issue.

I locally compiled the provider out of current branch and tested it like so:

➜  test_elasticstack_provider cat .terraform.rc 
provider_installation {
  dev_overrides {
    "registry.terraform.io/elastic/elasticstack" = "/home/alexandros/go/bin"
  }
  direct {}
}

➜  test_elasticstack_provider export TF_CLI_CONFIG_FILE=.terraform.rc 

➜  test_elasticstack_provider terraform plan
╷
│ Warning: Provider development overrides are in effect
│ 
│ The following provider development overrides are set in the CLI configuration:
│  - elastic/elasticstack in /home/alexandros/go/bin
│ 
│ The behavior may therefore not match any released version of the provider and applying changes may cause the state to become incompatible with published releases.
╵
data.elasticstack_kibana_action_connector.my-connector: Reading...

Planning failed. Terraform encountered an error while generating this plan.

╷
│ Error: error while creating elasticstack_kibana_action_connector datasource: connector with name [default/doesnotexist] and type [.pagerduty] not found
│ 
│   with data.elasticstack_kibana_action_connector.my-connector,
│   on main.tf line 1, in data "elasticstack_kibana_action_connector" "my-connector":
│    1: data "elasticstack_kibana_action_connector" "my-connector" {
│ 
╵

Now it errors out normally.

[tobio]

LGTM 😍