macOS: Handle missing key in QKeySequencePrivate::decodeString()

On macOS we pull out all the modifiers up front, which in the case
of incomplete key sequences such as "Meta+Shift+" will result in
an empty string as a result. The cross-platform code does not
handle that case, so we need to exit early.

This fixes an assert in tst_QKeySequence::parseString().

An assert has been added to QKeySequencePrivate::decodeString()
to make the assumption about the 'accel' argument explicit.

Change-Id: I135e62f9051a8b899202e5fb224b5d3c77bf2062
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

src/gui/kernel/qkeysequence.cpp

@@ -1062,6 +1062,8 @@ int QKeySequence::decodeString(const QString &str)
 
 int QKeySequencePrivate::decodeString(QString accel, QKeySequence::SequenceFormat format)
 {
+    Q_ASSERT(!accel.isEmpty());
+
     int ret = 0;
     accel = std::move(accel).toLower();
     bool nativeText = (format == QKeySequence::NativeText);
@@ -1121,7 +1123,10 @@ int QKeySequencePrivate::decodeString(QString accel, QKeySequence::SequenceForma
             sl = accel;
         }
     }
+    if (accel.isEmpty()) // Incomplete, like for "Meta+Shift+"
+        return Qt::Key_unknown;
 #endif
+
     int i = 0;
     int lastI = 0;
     while ((i = sl.indexOf(QLatin1Char('+'), i + 1)) != -1) {