connectivity test: avoid segfault if no agent pods are found

[multi-io]

If the --cilium-namespace and --agent-pod-selector (default kube-system and k8s-app=cilium) don't match anything, cilium connectivity test segfaults.

# (cluster where the agent pods are labeled `app=cilium)
# ./cilium connectivity test
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x2 addr=0x0 pc=0x102fbe948]

goroutine 24 [running]:
github.com/cilium/cilium-cli/connectivity/check.(*ConnectivityTest).detectCiliumVersion(0x140004ace08, {0x104e88238?, 0x140002f9f40?})
        /Users/oklischat/src/cilium-cli/connectivity/check/features.go:266 +0x168
github.com/cilium/cilium-cli/connectivity/check.(*ConnectivityTest).SetupAndValidate(0x140004ace08, {0x104e88238, 0x140002f9f40}, {0x12e860658, 0x10736f180})
        /Users/oklischat/src/cilium-cli/connectivity/check/context.go:290 +0xa4
github.com/cilium/cilium-cli/connectivity.Run({0x104e88238, 0x140002f9f40}, 0x140004ace08, {0x12e860628, 0x10736f180})
        /Users/oklischat/src/cilium-cli/connectivity/suite.go:24 +0x64
github.com/cilium/cilium-cli/cli.newCmdConnectivityTest.RunE.func1.2()
        /Users/oklischat/src/cilium-cli/cli/connectivity.go:99 +0x98
created by github.com/cilium/cilium-cli/cli.newCmdConnectivityTest.RunE.func1 in goroutine 1
        /Users/oklischat/src/cilium-cli/cli/connectivity.go:97 +0x5a4

This fixes that to a proper error.

# ./cilium connectivity test
connectivity test failed: no cilium agent pods found in -n kube-system -l k8s-app=cilium

[maintainer-s-little-helper]

Commit 4f63005 does not match "Signed-off-by".

Please follow instructions provided in https://docs.cilium.io/en/stable/contributing/development/contributing_guide/#developer-s-certificate-of-origin

[derailed]

@multi-io Great catch! Thank you for this pr.