Prevent the user flag computed fields crashes with anonymous users

Problem/Motivation

The flag module throws exceptions when the user requesting the flags is an anonymous user:

• Flag counts manager getUserFlagFlaggingCount (https://git.drupalcode.org/project/flag/-/blob/8.x-4.x/src/FlagCountMana...) method. it is used by ComutedFlagUserFlaggings (https://git.drupalcode.org/project/jsonapi_flag/-/blob/8.x-1.x/src/Plugi...) computed field.
• Flag service getAllEntityFlaggings (https://git.drupalcode.org/project/flag/-/blob/8.x-4.x/src/FlagService.p...) method. It is used by ComputedFlagginsCurrentUser (https://git.drupalcode.org/project/jsonapi_flag/-/blob/8.x-1.x/src/Plugi...) computed field.

This makes these 2 computed fields works only when the user is authenticated.

Steps to reproduce

Go to a JSON API endpoint that uses some of these computed fields with an anonymous user.

Proposed resolution

Capture the exceptions, and return empty values in the computed fields in these cases.