Fix environment call for failing tests

autoapi/mappers/dotnet.py

@@ -32,7 +32,8 @@ class DotNetSphinxMapper(SphinxMapperBase):
                     stdout=subprocess.PIPE,
                     stderr=subprocess.PIPE,
                     env=dict((key, os.environ[key])
-                              for key in ['PATH', 'DNX_PATH', 'HOME']))
+                             for key in ['PATH', 'DNX_PATH', 'HOME']
+                             if key in os.environ))
                 _, error_output = proc.communicate()
                 if error_output:
                     self.app.warn(error_output)