Genius: handle lyrics divs with no text

[valrus]

Description

Fixes #5583.

It's possible for a Genius page to have a lyrics div with no text that will be retrieved by BeautifulSoup's get_text method, which causes lyric fetching to crash. Based on the example in #5583 I believe the problem URL was this one: https://genius.com/Ludwig-van-beethoven-symphony-no-5-in-c-minor-op-67-1st-movement-allegro-con-brio-annotated
Note that the "lyrics" are a series of pictures of the pages of the score, so the text retrieved is an empty string. Prior to this fix, when this happened, this loop would remove the last character of the string until it was empty and then attempt to access the last character again and crash:

        while lyrics[-1] == "\n":
            lyrics = lyrics[:-1]

The intention of that loop seems to be to do what lyrics.rstrip('\n') would do. I replaced it with lyrics.rstrip() to ensure that if we have lyrics consisting only of any whitespace, they'll be turned into an empty string (and this will become None as in other cases where there are no lyrics to be found).

• Documentation: N/A, no change to interface or intended behavior
• Changelog: will do once approved
• Tests.

[github-actions]

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