Fix assert in QFontComboBox when setting empty font

Don't crash if the font does not have any families set.

Pick-to: 6.2
Task-number: QTBUG-97995
Change-Id: I8dc2f2fc00309b6fff6d4a661ec6d659f30808af
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Konstantin Ritt <ritt.ks@gmail.com>

src/widgets/widgets/qfontcombobox.cpp

@@ -371,7 +371,8 @@ void QFontComboBoxPrivate::_q_updateModel()
 void QFontComboBoxPrivate::_q_currentChanged(const QString &text)
 {
     Q_Q(QFontComboBox);
-    if (currentFont.families().first() != text) {
+    QStringList families = currentFont.families();
+    if (families.isEmpty() || families.first() != text) {
         currentFont.setFamilies(QStringList{text});
         emit q->currentFontChanged(currentFont);
     }

tests/auto/widgets/widgets/qfontcombobox/tst_qfontcombobox.cpp

@@ -29,6 +29,7 @@
 
 #include <QTest>
 #include <QSignalSpy>
+#include <QFontDatabase>
 
 #include <qfontcombobox.h>
 
@@ -49,6 +50,7 @@ private slots:
     void writingSystem_data();
     void writingSystem();
     void currentFontChanged();
+    void emptyFont();
 };
 
 // Subclass that exposes the protected functions.
@@ -277,6 +279,36 @@ void tst_QFontComboBox::currentFontChanged()
         qWarning("Not enough fonts installed on test system. Consider adding some");
 }
 
+void tst_QFontComboBox::emptyFont()
+{
+    QFontComboBox fontCB;
+    if (fontCB.count() < 2)
+        QSKIP("Not enough fonts on system to run test.");
+
+    QFont font;
+    font.setFamilies(QStringList());
+
+    // Due to QTBUG-98341, we need to find an index in the family list
+    // which does not match the default index for the empty font, otherwise
+    // the font selection will not be properly updated.
+    {
+        QFontInfo fi(font);
+        QStringList families = QFontDatabase::families();
+        int index = families.indexOf(fi.family());
+        if (index < 0)
+            index = 0;
+        if (index > 0)
+            index--;
+        else
+            index++;
+
+        fontCB.setCurrentIndex(index);
+    }
+
+    fontCB.setCurrentFont(font);
+    QVERIFY(!fontCB.currentFont().families().isEmpty());
+}
+
 QTEST_MAIN(tst_QFontComboBox)
 #include "tst_qfontcombobox.moc"