fix: return invalid step type (#1157)

If a step does not have either a `run` or `uses` directive it is
considered invalid.

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

pkg/model/workflow.go

@@ -414,6 +414,10 @@ const (
 
 // Type returns the type of the step
 func (s *Step) Type() StepType {
+	if s.Run == "" && s.Uses == "" {
+		return StepTypeInvalid
+	}
+
 	if s.Run != "" {
 		if s.Uses != "" {
 			return StepTypeInvalid