fix: update condition in account deletion function to prevent early return

[ggurdin]

Hello! The current check for mxids.length != 1 prevents account deletion from going through (I think this is left over from before the switch away from the adaptive_dialog package). I replaced this with a check for an empty string.

Thank you so much for your contribution to FluffyChat ❤️❤️❤️

Please make sure that your Pull Request meet the following acceptance criteria:

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[krille-chan]

Ah thank you so much <3