Debugger: Fix warnings on qt creator startup

Predefined entries like "Locals" do not have a value.

Remove asserts on every startup:

SOFT ASSERT: "!result.isEmpty()" in file
/home/kkoehne/dev/creator-2.8/src/plugins/debugger/watchhandler.cpp,
line 956

Change-Id: I16b97c61256c6083440b9fa228113be9d0867e5d
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: hjk <hjk121@nokiamail.com>