Fix [Bug]: Wrong `null` tagging in ManagedServiceIdentity

[MaryGao]

fixes https://github.com/Azure/typespec-azure/issues/1110

to match with the swagger definition https://github.com/Azure/azure-rest-api-specs/blob/87a08b955c257c773a3bd42553c718d4b1092955/specification/common-types/resource-management/v5/managedidentity.json#L9-L16

[azure-sdk]

All changed packages have been documented.

• :white_check_mark: @azure-tools/typespec-azure-resource-manager

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### `@azure-tools/typespec-azure-resource-manager` - _fix_ [✏️](https://github.com/Azure/typespec-azure/edit/fix-managed-identity-def-issue/.chronus/changes/fix-managed-identity-def-issue-2024-6-2-18-48-2.md?pr=/Azure/typespec-azure/pull/1111) > Fix the type discrepancy issue for property userAssignedIdentities in common types V5 ManagedServiceIdentity

[azure-sdk]

You can try these changes at https://cadlplayground.z22.web.core.windows.net/typespec-azure/prs/1111/

Check the website changes at https://tspwebsitepr.z22.web.core.windows.net/typespec-azure/prs/1111/

[timotheeguerin]

Can you regenerate the new common types and add the command to the regen-samples so it’s verified in the ci