engine: resources: Add another validation check

This simple check should prevent some silly mistakes and make the logic
easier for other parts of the code that won't have to worry about this
pattern.

engine/resources/file.go

@@ -130,6 +130,10 @@ func (obj *FileRes) Validate() error {
 		return fmt.Errorf("the State is invalid")
 	}
 
+	if obj.State == FileStateAbsent && obj.Content != nil {
+		return fmt.Errorf("can't specify content for an absent file")
+	}
+
 	if obj.Mode != "" {
 		if _, err := obj.mode(); err != nil {
 			return err