Fix crash when replacing \n around empty lines

Summary:
The problem is that QString("").startsWith("") returns false.
Similarly, QString("").endsWith("") also returns false.
However, the code previously was assuming that both cases return true.

This is now fixed, and verified through unit tests.

BUG: 381080

This deprecates patch D6473

Test Plan: make && make test

Reviewers: jsalatas, ltoscano, kfunk, cullmann, anthonyfieroni, mwolff

Reviewed By: cullmann

Subscribers: kwrite-devel, kde-frameworks-devel

Tags: #kate, #frameworks

Differential Revision: https://phabricator.kde.org/D14847