Update command should allow for deleting optional fields

[marcus-ny]

Description: The update command should allow users to delete optional fields such as next of kin.

Precondition: Contact Adam exists with just phone number and email, as shown in screenshot.

Steps to reproduce: 1.update u/Adam k/Charlie (succeeds as shown here)

2. update u/Adam k/

Notes: Although the UG has stated that the update fields cannot be empty, there is no other command in the app that allows for easily removing optional parameters. The only way is to delete the contact itself and add it again with the desired parameters. If for instance, next of kin was attached wrongly (intended for another person perhaps), and we want to remove the next of kin from Adam, we are not able to do so easily through update command.

[soc-se-bot]

Your response not required for this bug as the team has accepted the bug as it is.