Handle a warning location which evaluates to False

As written in PR #2856, this code isn't quite right.

An ArgumentNode object can evaluate as False (if it's arguments attribute is
an empty sequence).

If that happens, we then try to hand the location kwarg down to print(),
rather than removing it, which causes an invalid keyword argument exception.

This failure can be seen e.g. when generating for gnome-twitch (See [1])

[1] https://travis-ci.org/jon-turney/meson-corpus-test/jobs/343017109

mesonbuild/mlog.py

@@ -107,7 +107,7 @@ def warning(*args, **kwargs):
 
     args = (yellow('WARNING:'),) + args
 
-    if kwargs.get('location'):
+    if 'location' in kwargs:
         location = kwargs['location']
         del kwargs['location']
         location = '{}:{}:'.format(os.path.join(location.subdir, environment.build_filename), location.lineno)