identity: add a null value support for user assigned identity

[wuxu92]

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Description

Some resources support update/delete user identity by setting the value of the id to null, This PR is to add this ability by introduce a global variable for null value.

this is required by hashicorp/terraform-provider-azurerm#30293 and other resources.

Now to remove an user-assigned-identity, we can set like below:

		expandedIdentity, err := identity.ExpandUserAssignedMapFromModel(model.Identity)
		if err != nil {
			return fmt.Errorf("expanding `identity`: %+v", err)
		}
		if currentIdentity := exists.Model.Identity; currentIdentity != nil {
			// mark identity should be delete as null
			for id := range currentIdentity.IdentityIds {
				if _, ok := expandedIdentity.IdentityIds[id]; !ok {
					expandedIdentity.IdentityIds[id] = identity.NullUserAssignedIdentityDetails
				}
			}
		}

test:

--- PASS: TestUserAssignedIdentityDetails_MarshalJSON (0.00s)
    --- PASS: TestUserAssignedIdentityDetails_MarshalJSON/empty_identity (0.00s)
    --- PASS: TestUserAssignedIdentityDetails_MarshalJSON/nil_identity (0.00s)
    --- PASS: TestUserAssignedIdentityDetails_MarshalJSON/other_value_identity (0.00s)
    --- PASS: TestUserAssignedIdentityDetails_MarshalJSON/other_empty_identity (0.00s)
PASS

Change Log

Below please provide what should go into the changelog (if anything) conforming to the Changelog Format documented here.

• added null value support for user assigned identity

This is a (please select all that apply):

• Bug Fix
• New Feature
• Enhancement
• Breaking Change

Related Issue(s)

Rollback Plan

If a change needs to be reverted, we will publish an updated version of the provider.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

Note

If this PR changes meaningfully during the course of review please update the title and description as required.