Fix delete workspace-member avatar (#9630)

## Context
avatarUrl is a TEXT field type so non nullable, which means if we try to
run a mutation with null it will either fail or be ignored. Here this is
the second option, done in sanitizeRecordInput where when a
fieldMetadata has isNullable=false and the value is null, we return
undefined. This caused the mutation to send an empty input and not
remove the avatar

packages/twenty-front/src/modules/settings/profile/components/ProfilePictureUploader.tsx

@@ -91,7 +91,7 @@ export const ProfilePictureUploader = () => {
       await updateOneRecord({
         idToUpdate: currentWorkspaceMember?.id,
         updateOneRecordInput: {
-          avatarUrl: null,
+          avatarUrl: '',
         },
       });