Fix WebVTT ingestion when file does not specify start time

src/ctp_slack_bot/models/webvtt.py

@@ -77,7 +77,7 @@ class WebVTTContent(Content):
                         for result
                         in map(ISO_DATE_TIME_PATTERN.findall, web_vtt.header_comments)
                         if result)
-        except ValueError:
+        except (StopIteration, ValueError):
             return None
 
     @classmethod