QtCore: use new qEnvironmentVariableIs{Set,Empty}()

QtCore: use new qEnvironmentVariableIs{Set,Empty}()

In particular, qEmergencyOut() is now completely exception-free.

Incidentally, this patch shows that Qt isn't consistent in how it
treats empty environment variables used as flags, but that is something
for a separate commit. This patch aims to be behaviour-preserving,
except in exceptional circumstances, of course.

Change-Id: Ie106e7b430e1ab086c40c81cc1e56cd0e5400cb4
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>