fix(client): A2ACardResolver.get_agent_card will auto-populate with agent_card_path when relative_card_path is empty (#508)

- Previously, it would only override with the default path when
`relative_card_path` is `None`
- Developer Experience Improvement when `urlparse` or similar libraries
are used.

Author: holtskinner

src/a2a/client/card_resolver.py

@@ -53,7 +53,7 @@ async def get_agent_card(
         Args:
             relative_card_path: Optional path to the agent card endpoint,
                 relative to the base URL. If None, uses the default public
-                agent card path.
+                agent card path. Use `'/'` for an empty path.
             http_kwargs: Optional dictionary of keyword arguments to pass to the
                 underlying httpx.get request.
 
@@ -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: