Auto merge of #13107 - UK992:tidy, r=Wafflespeanut

Tidy: Handle empty files

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

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python/tidy/servo_tidy/tidy.py

@@ -739,6 +739,9 @@ def collect_errors_for_files(files_to_check, checking_functions, line_checking_f
             continue
         with open(filename, "r") as f:
             contents = f.read()
+            if not contents.strip():
+                yield filename, 0, "file is empty"
+                continue
             for check in checking_functions:
                 for error in check(filename, contents):
                     # the result will be: `(filename, line, message)`

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])