Fix bugs that may occur in the file_to_subtitles function

[Hupanbdfx]

@convert_path_to_string("filename")
def file_to_subtitles(filename, encoding=None):
"""Converts a srt file into subtitles.

The returned list is of the form ``[((start_time,end_time),'some text'),...]``
and can be fed to SubtitlesClip.

Only works for '.srt' format for the moment.
"""
times_texts = []
current_times = None
current_text = ""
with open(filename, "r", encoding=encoding) as file:
    for line in file:
        times = re.findall("([0-9]*:[0-9]*:[0-9]*,[0-9]*)", line)
        if times:
            current_times = [convert_to_seconds(t) for t in times]
        elif line.strip() == "":
            times_texts.append((current_times, current_text.strip("\n")))
            current_times, current_text = None, ""
        elif current_times:
            current_text += line
    # Fix the issue where the subtitle is not added to times_texts when it is the last line in an SRT file and the loop ends.
    if current_times is not None:
        times_texts.append((current_times, current_text.strip("\n")))
return times_texts

[JPLeBreton]

Another bug I noticed can happen during this function is that the elif line.strip() == "": block can add a (time, text) tuple to the list where time is None, which will always cause a ValueError. Simply checking if current_times is None before appending will avoid that.

Cases I found where this could happen include multiple subtitles in a row that are a blank line.

[keikoro]

@JPLeBreton Are you asking @Hupanbdfx to adapt their PR? If there's something else that needs fixing, it shouldn't get lost in the depths of a PR comment chain.