Fix VTT issues

openformats/formats/vtt.py

@@ -45,7 +45,9 @@ def parse(self, content, **kwargs):
         self.transcriber.copy_until(len(source))
 
         template = self.transcriber.get_destination()
-        if not template.startswith("WEBVTT"):
+        if len(stringset) == 0:
+            raise ParseError("There are no strings to translate")
+        elif not template.startswith("WEBVTT"):
             raise ParseError("VTT file should start with 'WEBVTT'!")
         return template, stringset
 
@@ -102,9 +104,8 @@ def _parse_section(self, offset, section):
         # Content
         string_to_translate = "\n".join(src_strings[timings_index + 1 :])
         if string_to_translate == "":
-            raise ParseError(
-                f"Subtitle is empty on line {self.transcriber.line_number + 2}"
-            )
+            # Do not include an empty subtitle in the stringset
+            return None, None
 
         string = OpenString(
             timings,

openformats/tests/formats/vtt/test_vtt.py

@@ -56,16 +56,13 @@ def test_vtt_occurrences(self):
         _, stringset = self.handler.parse(source)
         self.assertEqual(stringset[0].occurrences, '00:01:28.797,00:01:30.297')
 
-    def test_missing_string(self):
+    def test_empty_subtitle(self):
         source = strip_leading_spaces("""WEBVTT
 
             1
             00:01:28.797 --> 00:01:30.297
         """)
-        self._test_parse_error(
-            source,
-            "Subtitle is empty on line 5"
-        )
+        self._test_parse_error(source, "There are no strings to translate")
 
     def test_full_and_short_timings(self):
         source = strip_leading_spaces("""WEBVTT