Stop filtering out falsy environment values

Fixes #175

Author: crazymerlyn

pytest_html/plugin.py

@@ -466,7 +466,7 @@ def _generate_environment(self, config):
         environment = [html.h2('Environment')]
         rows = []
 
-        keys = [k for k in metadata.keys() if metadata[k]]
+        keys = [k for k in metadata.keys()]
         if not isinstance(metadata, OrderedDict):
             keys.sort()