Don't overwrite QMAKE_QT_CONFIG with empty value

If the mkspec sets QMAKE_QT_CONFIG, QMAKE_QT_CONFIG may be overwritten
with an empty value from .qmake.cache. Avoid this by first checking
if the value from .qmake.cache is not empty before assigning it to
QMAKE_QT_CONFIG.

This was already addressed differently in Qt 5.

Change-Id: I95fa630139b8798156a2fb15d0dde630a0a53a0a
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@digia.com>