Guard against empty keys in QPlatformInputContextFactory::create().

The code relied on QStringList::split() returning a list
consisting of one empty string when passing an enpty string.
Add a check to prevent the plugin loader from trying to load
in this case.

Change-Id: Iadb418d32fdea1d472d6c00726ad039b4afbf409
Reviewed-by: Andy Shaw <andy.shaw@theqtcompany.com>
Reviewed-by: Lars Knoll <lars.knoll@theqtcompany.com>

src/gui/kernel/qplatforminputcontextfactory.cpp

@@ -65,6 +65,7 @@ QString QPlatformInputContextFactory::requested()
 QPlatformInputContext *QPlatformInputContextFactory::create(const QString& key)
 {
 #if !defined(QT_NO_LIBRARY) && !defined(QT_NO_SETTINGS)
+    if (!key.isEmpty()) {
         QStringList paramList = key.split(QLatin1Char(':'));
         const QString platform = paramList.takeFirst().toLower();
 
@@ -74,6 +75,7 @@ QPlatformInputContext *QPlatformInputContextFactory::create(const QString& key)
             return ic;
 
         delete ic;
+    }
 #endif
     return 0;
 }