fix(boxsdkgen): fix RetryAfter function when there's no Retry-After header (box/box-codegen#903) (#1243)

Author: box-sdk-build

.codegen.json

@@ -1 +1 @@
-{ "engineHash": "3d9d391", "specHash": "31c41d5", "version": "4.2.0" }
+{ "engineHash": "f97463c", "specHash": "31c41d5", "version": "4.2.0" }

box_sdk_gen/networking/retries.py

@@ -90,7 +90,11 @@ def retry_after(
         fetch_response: FetchResponse,
         attempt_number: int,
     ) -> float:
-        retry_after_header: Optional[str] = fetch_response.headers.get('Retry-After')
+        retry_after_header: Optional[str] = (
+            fetch_response.headers.get('Retry-After')
+            if 'Retry-After' in fetch_response.headers
+            else None
+        )
         if not retry_after_header == None:
             return float(retry_after_header)
         randomization: float = random(