Minor spell check interface fixes

Summary:

• empty document does not resize bar widget when spell check is started without effect
• replacing the last word with an empty text does not leave the progress bar open
• show first suggestion in suggestion combo box
• clear replace-all-list when checking is finished

Test Plan:

• perform spell check on empty document
• replace trailing misspelled word in document with empty string
• recheck spelling of former "Replace All" word

Reviewers: KTextEditor, dhaumann

Reviewed By: KTextEditor, dhaumann

Subscribers: dhaumann, Frameworks

Tags: Kate, Frameworks

Differential Revision: https://phabricator.kde.org/D8154