Don't panic on missing command

[tkjaer]

This PR adds a check for missing command and returns help instead of exiting with a panic.

Prevents:

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

goroutine 27 [running]:
github.com/haproxytech/dataplaneapi/runtime.serve(0xc00033e0c8, {0x1edc2e0, 0xc00578e100})
	github.com/haproxytech/dataplaneapi/runtime/serve.go:38 +0x5a5
created by github.com/haproxytech/dataplaneapi/runtime.(*DebugServer).Start.func2 in goroutine 74
	github.com/haproxytech/dataplaneapi/runtime/runtime.go:102 +0x4e