Fixed the "remove existing mount point" bug

Summary:
The QString "contains method" is also searching for empty QStrings.
Due to this all entries in /etc/fstab were removed . After this patch it is
searching for only non-empty strings in removeMountPoint() function.

Reviewers: stikonas

Reviewed By: stikonas

Tags: KDE Partition Manager

Differential Revision: https://phabricator.kde.org/D11542