Allow find and replace with an empty string

[Shivix]

Currently if the replace field is empty, the write function will do nothing, and hitting enter will simply go through the occurrences replacing nothing.

Allowing the replace string to be empty, allows the user to use find and replace to rapidly erase text.

Since write specifically checks if string is empty, we could consider allowing an empty write, which does work in testing, but we can instead utilise extract_selection here instead. This is a bit more explicit and ensures this change only affects find and replace.

Closes #439

[lhecker]

I don't think this is the right approach. There's no inherent reason why write has to exit early for empty inputs. It's still meaningful if there's an active selection after all.

I've written an alternative approach over in #457 and I'd be happy if you could check it out.

[Shivix]

Yeah fair enough, allowing write to work with an empty input was actually what I went with at first. I had spent some time testing that also and it works well. I just went with the change that has lower potential impact to make it easier to review.

[lhecker]

#457 was just merged, so I'll close this PR for now. 🙂