Fix/dbt deps retry none answer (#4225)

* Fix issue #4178
Allow retries when the answer is None

* Include fix for #4178
Allow retries when the answer from dbt deps is None

* Add link to the PR

* Update exception and shorten line size

* Add test when dbt deps returns None

automatic commit by git-black, original commits:
  f20e83a

core/dbt/clients/registry.py

@@ -37,7 +37,7 @@ def _get(path, registry_base_url=None):
     # See https://github.com/dbt-labs/dbt-core/issues/4577
     if resp.json() is None:
         raise requests.exceptions.ContentDecodingError(
-            'Request error: The response is None', response=resp
+            "Request error: The response is None", response=resp
         )
     return resp.json()