fix: handle explicit null member defaults for enums and intEnums

[jonathan343]

Summary

Fixes a Python codegen crash when a structure member overrides an enum or intEnum default with an explicit @default(null).

Problem

A member can set @default(null) to mark itself nullable even when its target enum declares a non-null default. This is valid Smithy that appears in real AWS models (e.g. bedrock-agentcore-control's UpdatePolicyRequest$enforcementMode). In existing codegen, StructureGenerator.getDefaultValue sent these into the enum/intEnum branches, which read the default as a string/number without a null check and threw:

ExpectationNotMetException: Expected string, but found null.

Solution

Guard for a null default at the top of getDefaultValue so every non-document type resolves to None before the typed branches run (documents are excluded, their null default is a non-None Document(None)).

Added defaultNull{Enum,IntEnum,Blob,Timestamp} members to the integration model to cover this path.

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[arandito]