Fix runtime panic when config file is empty

[Furisto]

When the config file is not specified a runtime panic occcured, as
we tried to dereference the nil config object.

Signed-off-by: Thomas Schubart [email protected]

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x17d16ca]

goroutine 1 [running]:
main.hubbleTETRAGONExecute()
	/go/src/github.com/cilium/tetragon/cmd/tetragon/main.go:208 +0xa0a
main.execute.func1(0xc00011a780, {0x1d78f14, 0x1, 0x1})
	/go/src/github.com/cilium/tetragon/cmd/tetragon/main.go:314 +0x8a
github.com/spf13/cobra.(*Command).execute(0xc00011a780, {0xc000132010, 0x1, 0x1})
	/go/src/github.com/cilium/tetragon/vendor/github.com/spf13/cobra/command.go:856 +0x5f8
github.com/spf13/cobra.(*Command).ExecuteC(0xc00011a780)
	/go/src/github.com/cilium/tetragon/vendor/github.com/spf13/cobra/command.go:960 +0x3ad
github.com/spf13/cobra.(*Command).Execute(...)
	/go/src/github.com/cilium/tetragon/vendor/github.com/spf13/cobra/command.go:897
main.execute()
	/go/src/github.com/cilium/tetragon/cmd/tetragon/main.go:394 +0x9be
main.main()
	/go/src/github.com/cilium/tetragon/cmd/tetragon/tetragon.go:11 +0x19ru

[jrfastab]

@Furisto thanks I introduced this and also just hit it and had the same idea for a fix. So lets ship it.

@willfindlay Its valid to have a no config file, I use this for getting just the exec tracing running in some clusters. If we return an error from readConfig then we would need to capture the error and skip the next step or more such steps. Anyways I think this is reasonably clean as is. So I'm going to merge it. Feel free to continue the debate though if you have strong opinions.

[willfindlay]

@jrfastab yeah I realized that right after I left my comment. I still think there's probably a way to refactor this to be more idiomatic, but that can be done as follow up.