cli/command: PromptUserForCredentials: move "post" check for empty name

move the "post" check for username being empty inside the branch
that's handling the username, as it's the only branch where username
is mutated after checking if it's empty.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>
(cherry picked from commit 581cf36bd4)
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/command/registry.go

@@ -152,10 +152,11 @@ func PromptUserForCredentials(ctx context.Context, cli Cli, argUser, argPassword
 		if argUser == "" {
 			argUser = defaultUsername
 		}
-	}
 		if argUser == "" {
 			return registrytypes.AuthConfig{}, errors.Errorf("Error: Non-null Username Required")
 		}
+	}
+
 	if argPassword == "" {
 		restoreInput, err := DisableInputEcho(cli.In())
 		if err != nil {