custom_metadata not updated on entity-alias when it's the only given parameter

[maxiscoding28]

Describe the bug
A user is attempting to update only the custom_metadata field on an existing entity-alias.
https://developer.hashicorp.com/vault/api-docs/secret/identity/entity-alias#update-entity-alias-by-id

On version 1.9.2 the request throws a 500 error and the message "missing alias name".
On version 1.9.3 and above the request fails silently. The application indicates the updates was successful but the custom_metadata field isn't updated.

To Reproduce
Vault version 1.9.3 (and above, tested also on 1.12.2)

# Create a mount and save to a variable
vault auth enable -path=upass1 userpass
MA1=$(vault auth list -format=json | jq -r '."upass1/" | .accessor')

# Create an entity and save ID to variable
EID=$(vault write -field=id identity/entity name=max-winslow policies=entity-policy)

# Create an entity alias and save ID to variable
AID=$(vault write -field=id identity/entity-alias name=max1 canonical_id=$EID mount_accessor=$MA1)

# Read entity alias and verify that custom_metadata is empty
vault read identity/entity-alias/id/$AID

# Attempt to update only the custom_metadata field
vault write identity/entity-alias/id/$AID \
   - <<<'{"custom_metadata":{"foo":"bar"}}'
   
# Verify that the update failed silently by reading the entity-alias
vault read -field=custom_metadata identity/entity-alias/id/$AID

map[]

# Update entity alias with any other valid parameter for the endpoint 
# Dooesn't have to be name and doesn't have to be a new value for the additional parameter
vault write identity/entity-alias/id/$AID \
    mount_accessor=$MA1 \
    - <<<'{"custom_metadata":{"foo":"bar"}}'
 
# Verify that custom_metadata has now been updated
vault read -field=custom_metadata identity/entity-alias/id/$AID

map[foo:bar]

Vault version 1.9.2

# Create a mount and save to a variable
vault auth enable -path=upass1 userpass
MA1=$(vault auth list -format=json | jq -r '."upass1/" | .accessor')

# Create an entity and save ID to variable
EID=$(vault write -field=id identity/entity name=max-winslow policies=entity-policy)

# Create an entity alias and save ID to variable
AID=$(vault write -field=id identity/entity-alias name=max1 canonical_id=$EID mount_accessor=$MA1)

# Read entity alias and verify that custom_metadata is empty
vault read identity/entity-alias/id/$AID

# Attempt to update only the custom_metadata field (fails with 500)
vault write identity/entity-alias/id/$AID \
   - <<<'{"custom_metadata":{"foo":"bar"}}'
   
Error writing data to identity/entity-alias/id/5b6add1d-a796-4fda-7f13-fee0a6885b44: Error making API request.
URL: PUT http://127.0.0.1:8200/v1/identity/entity-alias/id/5b6add1d-a796-4fda-7f13-fee0a6885b44
Code: 500. Errors:
* 1 error occurred:
       * missing alias name

# Update entity alias with any other valid parameter for the endpoint 
# Dooesn't have to be name and doesn't have to be a new value for the additional parameter
vault write identity/entity-alias/id/$AID \
    mount_accessor=$MA1 \
    - <<<'{"custom_metadata":{"foo":"bar"}}'
 
# Verify that custom_metadata has now been updated
vault read -field=custom_metadata identity/entity-alias/id/$AID    

Expected behavior
Updating only the custom_metadata field should complete successfully. Also, documentation indicates name is a required parameter however this parameter is not required for updating other fields on entity-alias and is not even exclusively required in order to update custom_metadata any other valid parameter will allow the update to succeed.

Environment:
Tested on 1.9.2, 1.9.3 and 1.12.2

Additional context
It seems likely this behavior is related to an improvement introduced in 1.9.3 around the custom_metadata parameter for identity alias
#13395