style: standardize property null checks to use ternary operators (#354)

# Description

## Summary
Standardizes property null check patterns across the codebase by
converting early return style to ternary operator style for consistency.

## Changes
- **RequestContext.configuration**: Converted from `if not self._params:
return None` pattern to ternary operator
- **RequestContext.metadata**: Converted from `if not self._params:
return {}` pattern to ternary operator

Author: vtigo

src/a2a/server/agent_execution/context.py

@@ -127,9 +127,7 @@ def context_id(self) -> str | None:
     @property
     def configuration(self) -> MessageSendConfiguration | None:
         """The `MessageSendConfiguration` from the request, if available."""
-        if not self._params:
-            return None
-        return self._params.configuration
+        return self._params.configuration if self._params else None
 
     @property
     def call_context(self) -> ServerCallContext | None:
@@ -139,9 +137,7 @@ def call_context(self) -> ServerCallContext | None:
     @property
     def metadata(self) -> dict[str, Any]:
         """Metadata associated with the request, if available."""
-        if not self._params:
-            return {}
-        return self._params.metadata or {}
+        return self._params.metadata or {} if self._params else {}
 
     def add_activated_extension(self, uri: str) -> None:
         """Add an extension to the set of activated extensions for this request.