Treat empty $MPLBACKEND as an unset value.

anntzer · anntzer · commit 9643a577fb74 · 2018-06-25T00:03:34.000+02:00
This allows one to temporarily unset the environment variable with
`MPLBACKEND= python ...`, and is consistent with standard behavior.
diff --git a/lib/matplotlib/__init__.py b/lib/matplotlib/__init__.py
@@ -1365,10 +1365,8 @@ def use(arg, warn=True, force=False):
         importlib.reload(sys.modules['matplotlib.backends'])
 
 
-try:
+if os.environ.get('MPLBACKEND'):
     use(os.environ['MPLBACKEND'])
-except KeyError:
-    pass
 
 
 def get_backend():
