Don't show an error when trying do paste while there is nothing to paste

Tobias Fellarequested to merge
work/tobias/no-error-empty-paste into master

The error would currently be shown in the form of an empty notification. IMO, it's not necessary in this case to show an error at all.

This also applies to mimetypes that would be filtered out at a later stage (e.g., various x-kde- hints that are meaningless in this context).

Also removes a slight inconsistency in which formats are filtered.

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