Fix Map cast to preserve nullable value types (#850)

* Initial plan

* Fix nullable Map value types in service generation

Co-authored-by: trevorwang <121966+trevorwang@users.noreply.github.com>

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Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: trevorwang <121966+trevorwang@users.noreply.github.com>

Author: Copilot