"empty translation" does not mean "no translation"

this makes QCoreApplication::translate() consistent with
QTranslator::translate(), and is semantically cleaner.
users wishing to shrink their QM files can do that properly by using
lrelease -removeidentical.

Change-Id: I2b367314cfb985c3d130c7c6347e2742311f497a
Reviewed-on: http://codereview.qt-project.org/5165
Reviewed-by: Qt Sanity Bot <qt_sanity_bot@ovi.com>
Reviewed-by: hjk <qthjk@ovi.com>

dist/changes-5.0.0

@@ -28,6 +28,9 @@ information about a particular change.
   preprocessor #define has also been removed. The build-key is obsolete
   and is no longer necessary.
 
+- QCoreApplication::translate() will no longer return the source text when
+  the translation is empty. Use lrelease -removeidentical for optimization.
+
 ****************************************************************************
 *                           General                                        *
 ****************************************************************************

src/corelib/kernel/qcoreapplication.cpp

@@ -1823,12 +1823,12 @@ QString QCoreApplication::translate(const char *context, const char *sourceText,
         for (it = self->d_func()->translators.constBegin(); it != self->d_func()->translators.constEnd(); ++it) {
             translationFile = *it;
             result = translationFile->translate(context, sourceText, disambiguation, n);
-            if (!result.isEmpty())
+            if (!result.isNull())
                 break;
         }
     }
 
-    if (result.isEmpty()) {
+    if (result.isNull()) {
 #ifdef QT_NO_TEXTCODEC
         Q_UNUSED(encoding)
 #else