Fix Find & Replace with empty replacements

[lhecker]

Closes #439

[Shivix]

One thing I noticed with this approach is that some of the callers of this function check if it's empty before calling it anyway. So most of the behaviour is the same outside of the find & replace use case.
The exception to that is the following:

edit/src/bin/edit/draw_menubar.rs

Line 81 in f0cb63b

tb.write(ctx.clipboard(), true);

Edit:
On further testing, this changes the behaviour here, we now will delete any selection if we press the paste menu button with an empty clipboard.

[lhecker]

Oh, thanks for noticing this.

[lhecker]

On second thought, coincidentally we don't need to fix this, because I already rewrote clipboard handling where it checks for empty clipboard contents: #405