Relax REST client /v1/config response constraints (apache#2148)

fixing apache#2122

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# Rationale for this change

# Are these changes tested?
Yes tested locally

# Are there any user-facing changes?

Nope, just use them

---------

Co-authored-by: Kevin Liu <kevinjqliu@users.noreply.github.com>
Co-authored-by: Fokko Driesprong <fokko@apache.org>

Author: 3 people

pyiceberg/catalog/rest/__init__.py

@@ -178,8 +178,8 @@ class RegisterTableRequest(IcebergBaseModel):
 
 
 class ConfigResponse(IcebergBaseModel):
-    defaults: Properties = Field()
-    overrides: Properties = Field()
+    defaults: Optional[Properties] = Field(default_factory=dict)
+    overrides: Optional[Properties] = Field(default_factory=dict)
 
 
 class ListNamespaceResponse(IcebergBaseModel):