Cocoa: Handle keyboard dead keys correctly

Cocoa: Handle keyboard dead keys correctly

Some keyboard layouts have physical dead keys (like the ¨ key on a
Norwegian keyboard). These do not send any text, so we should not use
[NSString characterAtIndex:0] if the string is empty. When encountering
an empty [NSEvent character] string, use Qt::Key_unknown and
QChar::ReplacementCharacter.

Change-Id: I7281aa9ea6005341c0dcfa5900bfe601e4eac6a9
Reviewed-by: Morten Johan Sørvig <morten.sorvig@nokia.com>