fix bug we cannot use optional params

aiohttp_pydantic/utils.py

@@ -0,0 +1,11 @@
+from pydantic import BaseModel
+
+
+def is_pydantic_base_model(obj):
+    """
+    Return true is obj is a pydantic.BaseModel subclass.
+    """
+    try:
+        return issubclass(obj, BaseModel)
+    except TypeError:
+        return False