subparse: fix pushing WebVTT cue when last is not an empty line
If the last WebVTT cue does not have an empty line after it, or if it does not end with a newline at all, it does not get pushed out and it won't be displayed. gst_sub_parse_sink_event() already handles the issue for other subtitle formats, enable handling it for GST_SUB_PARSE_FORMAT_VTT too. While at it also add a test for this case.
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