Fix check for missing engine in comic applet

Summary:
Current code would check the comic engine was not installed and not create the
download dialog, but would still always try to show it. This would
crash.

BUG: 372313

Test Plan: Code review

Reviewers: #plasma

Subscribers: plasma-devel

Tags: #plasma

Differential Revision: https://phabricator.kde.org/D6746