Fail generateTests when testData directory is empty

When switching git branches, there might be some empty directories left
This causes generateTests to pass locally with no changes and fail on CI

generators/test-generator/tests/org/jetbrains/kotlin/generators/model/SimpleTestClassModel.kt

@@ -114,6 +114,13 @@ class SimpleTestClassModel(
                     if (file.isDirectory && excludeParentDirs && dirHasSubDirs(file)) {
                         continue
                     }
+                    if (file.isDirectory && !dirHasFilesInside(file)) {
+                        throw IllegalStateException(
+                            "testData directory $file is empty. " +
+                                    "This might be due to git branch switching removed the contents but left directory intact. " +
+                                    "Consider removing empty directory or revert removing of its' contents."
+                        )
+                    }
                     result.addAll(
                         methodModelLocator(
                             rootFile, file, filenamePattern,