fix: lint

Signed-off-by: Shingo OKAWA <shingo.okawa.g.h.c@gmail.com>

Author: ognis1205

src/a2a/client/auth/interceptor.py

@@ -49,8 +49,12 @@ async def intercept(
                     scheme_def_union = agent_card.securitySchemes.get(
                         scheme_name
                     )
+                    if not scheme_def_union:
+                        continue
                     scheme_def = scheme_def_union.root
+
                     headers = http_kwargs.get('headers', {})
+
                     match scheme_def:
                         # Case 1a: HTTP Bearer scheme with an if guard
                         case HTTPAuthSecurityScheme() if (