servo: Merge #13107 - Tidy: Handle empty files (from UK992:tidy); r=Wafflespeanut

Fix index error ``list index out of range``, when file is empty.

r? @Wafflespeanut

---
- [x] `./mach build -d` does not report any errors
- [x] `./mach test-tidy` does not report any errors
- [x] There are tests for these changes

Source-Repo: https://github.com/servo/servo
Source-Revision: c3ce9f2c909cd75c8f424bcef4036d0e29476546

servo/python/tidy/servo_tidy_tests/test_tidy.py

@@ -39,6 +39,11 @@ class CheckTidiness(unittest.TestCase):
         self.assertEqual('no newline at EOF', errors.next()[2])
         self.assertNoMoreErrors(errors)
 
+    def test_empty_file(self):
+        errors = tidy.collect_errors_for_files(iterFile('empty_file.rs'), [], [tidy.check_by_line], print_text=False)
+        self.assertEqual('file is empty', errors.next()[2])
+        self.assertNoMoreErrors(errors)
+
     def test_long_line(self):
         errors = tidy.collect_errors_for_files(iterFile('long_line.rs'), [], [tidy.check_by_line], print_text=False)
         self.assertEqual('Line is longer than 120 characters', errors.next()[2])