Account for a mnemonic label to go back to ""
Summary:
We are discarding the value if there's no weights which meant that when
setting a button to "", the text provided by mnemonicLabel wouldn't update.
This patch changes it so it only skips the weights computation if there's no
text.
Test Plan:
Tested with patch that removes the text of some buttons when the
screen is smaller on Discover.
Reviewers: Kirigami, mart, ngraham
Reviewed By: ngraham
Subscribers: plasma-devel
Tags: Kirigami
Differential Revision: https://phabricator.kde.org/D25618