Including max_combo of non-Standard gamemodes in /get_beatmaps

[MonoHearted]

With the recent addition of max_combo from beatmaps, all other gamemodes except standard (osu!mania, taiko, Catch the Beat) have their max_combo values set to null.

[Existanza]

I was surprised when my project didn't work; it turns out putting null as a value rather than something in the quotation marks confuses Python's ast.literal_eval function.