Fixed account_provider mandatory

Class member account_provider was mandatory due to being used when loading module PlanModule. Fixed by initializing it as None (it makes modules requiring it not functioning if not properly created)

Author: NAST0R

src/sentinel_sdk/sdk.py

@@ -29,7 +29,8 @@ def __init__(self, grpcaddr: str, grpcport: int, secret: str = None, ssl: bool =
 
         self._client = None
         self._account = None
-
+        self._provider_account = None
+        
         if secret is not None:
             self.__setup_account_and_client(grpcaddr, grpcport, secret, ssl)