Return transparent background color if no brush is set

Fixes: QTBUG-110751
Pick-to: 6.5
Change-Id: I6fa33ba22b0209da6c9e1022851f84c892a5ae5f
Reviewed-by: Eskil Abrahamsen Blomfeldt <eskil.abrahamsen-blomfeldt@qt.io>

src/widgets/widgets/qtextedit.cpp

@@ -713,7 +713,8 @@ QColor QTextEdit::textColor() const
 QColor QTextEdit::textBackgroundColor() const
 {
     Q_D(const QTextEdit);
-    return d->control->textCursor().charFormat().background().color();
+    const QBrush &brush = d->control->textCursor().charFormat().background();
+    return brush.style() == Qt::NoBrush ? Qt::transparent : brush.color();
 }
 
 /*!