Fix the nullability of snapshot-id on AssertRefSnapshotId (#103)

It is not super clear from the current structure:

https://github.com/apache/iceberg/blob/main/open-api/rest-catalog-open-api.py#L304-L329

This is because I had to deconstruct the object.
It would be great to get  apache/iceberg#7710 in

pyiceberg/table/__init__.py

@@ -374,7 +374,7 @@ class AssertRefSnapshotId(TableRequirement):
 
     type: Literal["assert-ref-snapshot-id"] = Field(default="assert-ref-snapshot-id")
     ref: str
-    snapshot_id: int = Field(..., alias="snapshot-id")
+    snapshot_id: Optional[int] = Field(default=None, alias="snapshot-id")
 
 
 class AssertLastAssignedFieldId(TableRequirement):