TestMsgServerUpdateMetadataAuthority suggestions

[giorgionocera]

In the nft module, in the nft/keeper/msg_server_test.go file, lays the TestMsgServerUpdateMetadataAuthority method. This method implements the tests for updating metadata authority

Here, it is possible to change authority also for immutable metadata as discussed in #200. We need to understand if it is normal behavior. Moreover, as expressed in #188 no check is done on addresses, so they could also be empty strings.

In the end, as discussed in point 3 of #159, we could analyze whether it would not be more convenient to remove the isMutable attribute and, use an address (isMutable = true) or an empty string (isMutable = false) for the same purpose.