Fix NullPointerException when no file name is given

This change ensures that Copybara handles cases where the configuration path
provided on the command line does not have a filename (e.g., the root directory
'/'), preventing a `java.lang.NullPointerException` from `Path.getFileName().toString()`.

BUG=485259576
GWSQ_IGNORE: chriscampos@google.com
PiperOrigin-RevId: 883360034
Change-Id: I5c413e51943b572029aef9b72d4679400a8a9dfe

Author: chris-campos

java/com/google/copybara/Main.java

@@ -414,11 +414,13 @@ protected ContextProvider newInfoContextProvider() {
   protected Path validateLocalConfig(GeneralOptions generalOptions, String configLocation)
       throws ValidationException {
     Path configPath = generalOptions.getFileSystem().getPath(configLocation).normalize();
-    String fileName = configPath.getFileName().toString();
+    Path fileName = configPath.getFileName();
+    checkCondition(fileName != null, "The configuration path '%s' is not a file.", configPath);
     checkCondition(
-        fileName.contentEquals(COPYBARA_SKYLARK_CONFIG_FILENAME),
+        fileName.toString().contentEquals(COPYBARA_SKYLARK_CONFIG_FILENAME),
         "Copybara config file filename should be '%s' but it is '%s'.",
-            COPYBARA_SKYLARK_CONFIG_FILENAME, configPath.getFileName());
+        COPYBARA_SKYLARK_CONFIG_FILENAME,
+        fileName);
 
     // Treat the top level element specially since it is passed thru the command line.
     if (!Files.exists(configPath)) {