Fix NPE in staging approve command

The class member variable 'emailMessages' was never properly allocated and this
caused NPE (null pointer exception) when accessing it.

Task-number: QTQAINFRA-905
Change-Id: I99d91a3345ca6ef8b05d4b0436929fff33d7e2da
Reviewed-by: Oswald Buddenhagen <oswald.buddenhagen@theqtcompany.com>