Cocoa: Make dictation via speech recognition work

Returning NSNotFound from the NSTextInputClient selectedRange
implementation when there is no selection prevents
dictation from activating (for unknown reasons).

Return an empty {0, 0} range instead. Text input
methods such as Pinyin still work after this change.

[ChangeLog][macOS] Speech to text dictation now works
for Qt text input.

Change-Id: Ibf1729bdd271e8ed5ce3c9d2a0373c8ab3613d8e
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@qt.io>

src/plugins/platforms/cocoa/qnsview.mm

@@ -1783,9 +1783,7 @@ static QTabletEvent::TabletDevice wacomTabletDevice(NSEvent *theEvent)
 
 - (NSRange) selectedRange
 {
-    NSRange selectedRange = {NSNotFound, 0};
-    selectedRange.location = NSNotFound;
-    selectedRange.length = 0;
+    NSRange selectedRange = {0, 0};
 
     QObject *fo = QGuiApplication::focusObject();
     if (!fo)