[5.x] Prevent error when Laravel Telescope serializes a user before its created

[duncanmcclean]

This pull request fixes an issue where an error would occur when creating a new user, when the Laravel Telescope package is installed.

• When a user is being created, it dispatches two events: UserCreating and UserSaving.
• Laravel Telescope listens for any events being dispatched and attempts to serialize them using json_encode.
• When the User object gets serialized, it'll end up augmenting the user via HasAugmentedInstance::jsonSerialize().
• The AugmentedUser::commonKeys() array includes edit_url and api_url.
• When the editUrl and apiUrl methods are called on the User, they error out due to no ID being set, since the user hasn't been created yet.
  • We do a similar "if no id, return null" check for editUrl and apiUrl in entries.
  • We don't do this check for things like taxonomies, because their URLs use slugs which exist on the object before the term is created.

Closes #10525.