engine: return nil from dry run when no changes

The `command` output actually relies on `nil` being return when there
are no changes, and I forgot to add that behaviour to dry run which made
there be no message printed out for confirmation.

Also decided to make a slight change to the confirmation message so that
it outputs nothing when the `-quiet` flag is enabled.

Signed-off-by: braydonk <braydonk@google.com>

command/command.go

@@ -146,8 +146,8 @@ func (c *Command) Run() error {
 		}
 		if out != nil {
 			fmt.Print(out)
-		} else {
-			fmt.Print("No files will be changed.")
+		} else if !c.Quiet {
+			fmt.Println("No files will be changed.")
 		}
 	case yamlfmt.OperationStdin:
 		stdinYaml, err := readFromStdin()

engine/consecutive_engine.go

@@ -63,6 +63,9 @@ func (e *ConsecutiveEngine) DryRun(paths []string) (fmt.Stringer, error) {
 	if len(formatErrs) > 0 {
 		return nil, formatErrs
 	}
+	if formatDiffs.ChangedCount() == 0 {
+		return nil, nil
+	}
 	return getEngineOutput(e.OutputFormat, yamlfmt.OperationDry, formatDiffs, e.Quiet)
 }