fix: fix generating pdf with empty subtitles

[EKarton]

Description:

Problem:

When we have a video like https://www.youtube.com/watch?v=KN6OSdUfgyA with subtitles like
subtitles_8-old.txt, we get an exception thrown:

Number of frames: 103
Getting subtitles for each frame
Traceback (most recent call last):
  File "src/main.py", line 87, in <module>
    runner.run(sys.argv[1:])
  File "src/main.py", line 51, in run
    self.__run__(
  File "src/main.py", line 70, in __run__
    segments = segment_finder.get_subtitle_segments(subtitle_breaks)
  File "D:\Study\Sixth-Sem\Lecture-Video-to-PDF\src\subtitle_segment_finder.py", line 42, in get_subtitle_segments
    pos = self.__get_part_position_of_time_break__(time_break)
  File "D:\Study\Sixth-Sem\Lecture-Video-to-PDF\src\subtitle_segment_finder.py", line 122, in __get_part_position_of_time_break__
    if self.parts[right_part_index].text[right_part_char_index] == ".":
IndexError: string index out of range```

Cause:

This is because the subtitle file contains subtitle parts that has no content in it. For instance,

This causes the check of self.parts[right_part_index].text[right_part_char_index] to fail since right_part_index is a valid index in self.parts but since the text is empty and we first set right_part_char_index to 0, it will cause self.parts[right_part_index].text[right_part_char_index] to fail.

Solution:

Each subtitle part needs to have some text. A subtitle part with no text is redundant. Hence, when we parse the subtitles, we need to remove the subtitle parts that have no text.

Manual Test:

1. Ran python3 src/main.py tests/videos/input_8.mp4 -s tests/subtitles/subtitles_8.1.vrt -o output.pdf. It did not crash, and it returned this PDF:

output.pdf

2. Unfortunately, the subtitle file is not the best, and the subtitle at tests/subtitles/subtitles_8.srt is a better replacement for that video's subtitle. Running python3 src/main.py tests/videos/input_8.mp4 -s tests/subtitles/subtitles_8.srt -o output.pdf` produced this subtitle:

output.pdf