WidgetColorizer: eliminate unsafe QStringView usage

4cbff15f changed the type of the text local variable from QString to QStringView. The underlying QString is a temporary returned by QTextFragment::text() that is destroyed at the end of the full expression. But the QStringView text is checked for emptiness on the next line.

Combine the QStringView construction and usage into a single line to make the usage safer. The code did not actually trigger undefined behavior, because only QStringView::isEmpty(), which accesses only QStringView::m_size, was called on the text variable. And the trivial QStringView destructor does not dereference the dangling pointer QStringView::m_data either.

Fixes #52 (closed).