#604 Add existence and type check for value passed to computed prop

[crablab]

Adds existence and type checking for the val parameter passed to the computed prop, to handle the case that a null value is passed.

[warrensearle]

@crablab can you double check the required behaviour here.
This change means that user will be unable to empty a value from the 'estimated launch date' field.
I would expect emptying the month and year to be a save-able change

[crablab]

@warrensearle Ah, it was my understanding that this should not be possible.

To confirm, we want to allow a null value to be saved? In which case, we just need to handle that case rather than trying to transform a null value into a date 👍

[warrensearle]

Providing we're happy that once a user has provided a launch date they can't remove it then this is good to go

[crablab]

@ClaireTroughtonJAC Would you be able to just confirm the understanding here either way?

Understanding: Once a launch date has been provided (eg. when the Exercise is created) it can be modified, but it cannot be set to null (ie. no launch date).

[crablab]

Claire has confirmed this is correct 👍