Fix isNull check for RegularArea

Summary:
First check in RegularArea::isNull sais this: "I'm a QList. If I'm empty, I'm not null". That doesn't make sense, invert it.

The next check says: "If I contain at least one shape which is not null, I'm not null.". That makes sense, keep it.

Test Plan: - check for regression in callers (TextPage::findText, TextPage::text, TextPage::words)

Reviewers: aacid

Reviewed By: aacid

Subscribers: okular-devel

Tags: #okular

Differential Revision: https://phabricator.kde.org/D15344