Don't create empty backups

Not really a problem, but created unnecessary files and changes existing
behavior.

state/backup.go

@@ -1,8 +1,6 @@
 package state
 
-import (
-	"github.com/hashicorp/terraform/terraform"
-)
+import "github.com/hashicorp/terraform/terraform"
 
 // BackupState wraps a State that backs up the state on the first time that
 // a WriteState or PersistState is called.
@@ -68,9 +66,14 @@ func (s *BackupState) backup() error {
 		state = s.Real.State()
 	}
 
-	ls := &LocalState{Path: s.Path}
-	if err := ls.WriteState(state); err != nil {
-		return err
+	// LocalState.WriteState ensures that a file always exists for locking
+	// purposes, but we don't need a backup or lock if the state is empty, so
+	// skip this with a nil state.
+	if state != nil {
+		ls := &LocalState{Path: s.Path}
+		if err := ls.WriteState(state); err != nil {
+			return err
+		}
 	}
 
 	s.done = true