fix(client): A2ACardResolver.get_agent_card will auto-populate with agent_card_path when relative_card_path is empty

- Previously, it would only override with the default path when `relative_card_path` is None

Author: holtskinner

src/a2a/client/card_resolver.py

@@ -65,7 +65,7 @@ async def get_agent_card(
             A2AClientJSONError: If the response body cannot be decoded as JSON
                 or validated against the AgentCard schema.
         """
-        if relative_card_path is None:
+        if not relative_card_path:
             # Use the default public agent card path configured during initialization
             path_segment = self.agent_card_path
         else: