macOS: Return invalid QColor if qt_mac_toQColor is called with a nil NSColor

Call sites of qt_mac_toQColor may not necessarily check the NSColor
for nil before passing it to qt_mac_toQColor, as seen for example
in the QCocoaColorPicker implementation.

Without an explicit nil check we ended up returning a valid QColor
with all color components set to 0.

Pick-to: 6.7
Change-Id: I0cc36049fca05a6b668c0c610bc40eedbc72df3a
Reviewed-by: Doris Verria <doris.verria@qt.io>
(cherry picked from commit 271bfe4a5405309dc8d48807d6f28d174efb2cc2)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/gui/painting/qcoregraphics.mm

@@ -228,6 +228,9 @@ QColor qt_mac_toQColor(CGColorRef color)
 #ifdef Q_OS_MACOS
 QColor qt_mac_toQColor(const NSColor *color)
 {
+    if (!color)
+        return QColor();
+
     QColor qtColor;
     switch (color.type) {
     case NSColorTypeComponentBased: {