Merge pull request #4 from mickael-mounier/fix-python2-CalledProcessE…

…rror-None-output

Fix Python2 CalledProcessError exception unhandled None value

src/pytest_docker/__init__.py

@@ -18,7 +18,7 @@ def execute(command, success_codes=(0,)):
         )
         status = 0
     except subprocess.CalledProcessError as error:
-        output = error.output
+        output = error.output or b''
         status = error.returncode
         command = error.cmd
     output = output.decode('utf-8')