Check for empty style returned from environment

Fix warning
"Empty filename passed to function"
when running Qt Creator 4.4 against Qt 5.10
(see qtbase: 0dee566e98f5ff4f224e596de1c04de4f9685df4).

Change-Id: I0b71a873245a28ef972c105e7d41347e6b0fb38a
Reviewed-by: J-P Nurmi <jpnurmi@qt.io>