Fix crash when passed --only with a configured rule

[wata727]

Similar to #1105, it is crashed when passed --only with a configured rule.

% tflint --only aws_s3_bucket_name
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x18 pc=0x186a197]

goroutine 38 [running]:
github.com/terraform-linters/tflint/tflint.(*RuleConfig).Bytes(...)
        /Users/watanabekazuma/workspace/golang/src/github.com/wata727/tflint/tflint/config.go:430
github.com/terraform-linters/tflint/plugin.(*Server).RuleConfig(0xc00014de30, 0xc00017d7b0, 0xc0006ece70, 0x0, 0x0)
        /Users/watanabekazuma/workspace/golang/src/github.com/wata727/tflint/plugin/server.go:167 +0x1b7
reflect.Value.call(0xc0007056e0, 0xc00015cc80, 0x13, 0x2735c99, 0x4, 0xc000613f08, 0x3, 0x3, 0x26cf760, 0xc0004ac000, ...)
        /usr/local/Cellar/go/1.16/libexec/src/reflect/value.go:476 +0x8e7
reflect.Value.Call(0xc0007056e0, 0xc00015cc80, 0x13, 0xc000124708, 0x3, 0x3, 0x2a2d660, 0x100010000, 0xc00068d040)
        /usr/local/Cellar/go/1.16/libexec/src/reflect/value.go:337 +0xb9
net/rpc.(*service).call(0xc000702dc0, 0xc000701c70, 0xc0001406e0, 0xc0001406f0, 0xc000158780, 0xc0000509e0, 0x2213760, 0xc0
0017d7b0, 0x16, 0x22137a0, ...)
        /usr/local/Cellar/go/1.16/libexec/src/net/rpc/server.go:377 +0x189
created by net/rpc.(*Server).ServeCodec
        /usr/local/Cellar/go/1.16/libexec/src/net/rpc/server.go:474 +0x44d