Bugfix: proper plugin configuration was reset

Before this patch, if config was ['/some/path'] then it would enter the
else block and config would be set to [].

The regression this patch fixes was introduced by 700db154.

lib/ansible/plugins/__init__.py

@@ -66,7 +66,7 @@ class PluginLoader:
 
         if config and not isinstance(config, list):
             config = [config]
-        else:
+        elif not config:
             config = []
 
         self.config = config

test/units/plugins/test_plugins.py

@@ -75,3 +75,7 @@ class TestErrors(unittest.TestCase):
         #with patch('glob.glob', mock_glob):
         #    pass
 
+    def test_plugin__config(self):
+        config = ['/one', '/two']
+        pl = PluginLoader('test', '', config, 'test_plugin')
+        self.assertEqual(pl.config, config)