realm_user event: Fix custom_profile_field.value to be string | null

See zulip/zulip#22103, which I sent after
observing that this was sometimes null.

src/api/eventTypes.js

@@ -350,7 +350,7 @@ type PersonCommon =
   | SubsetProperties<UserOrBot, {| +user_id: mixed, +delivery_email: mixed |}>
   | {|
       +user_id: $PropertyType<UserOrBot, 'user_id'>,
-      +custom_profile_field: {| +id: number, +value: string, +rendered_value?: string |},
+      +custom_profile_field: {| +id: number, +value: string | null, +rendered_value?: string |},
     |}
   | {| +user_id: $PropertyType<UserOrBot, 'user_id'>, +new_email: string |};
 

src/users/__tests__/usersReducer-test.js

@@ -135,6 +135,10 @@ describe('usersReducer', () => {
       );
     });
 
+    test('When the user clears one of their custom profile fields.', () => {
+      check({ custom_profile_field: { id: 4, value: null } }, { ...theUser, profile_data: {} });
+    });
+
     test('When the Zulip display email address of a user changes', () => {
       const new_email = randString();
       check({ new_email }, { ...theUser, email: new_email });

src/users/usersReducer.js

@@ -47,13 +47,29 @@ export default (
                 if (person.custom_profile_field) {
                   return {
                     ...user,
-                    profile_data: {
-                      ...(user.profile_data: $PropertyType<User, 'profile_data'>),
-                      [person.custom_profile_field.id]: {
-                        value: person.custom_profile_field.value,
-                        rendered_value: person.custom_profile_field.rendered_value,
-                      },
-                    },
+                    profile_data: (() => {
+                      if (person.custom_profile_field.value !== null) {
+                        return {
+                          ...(user.profile_data: $PropertyType<User, 'profile_data'>),
+                          [person.custom_profile_field.id]: ({
+                            value: person.custom_profile_field.value,
+                            rendered_value: person.custom_profile_field.rendered_value,
+
+                            // FlowIssue: This assertion is cumbersome. But
+                            // it fills a gap in Flow's coverage…that
+                            // apparently Flow doesn't announce by marking
+                            // anything with `any`. Remove when doing so
+                            // doesn't stop Flow from catching something
+                            // wrong on `value` or `rendered_value`.
+                          }: $Values<$NonMaybeType<$PropertyType<User, 'profile_data'>>>),
+                        };
+                      } else {
+                        // eslint-disable-next-line no-unused-vars
+                        const { [person.custom_profile_field.id.toString()]: _, ...rest } =
+                          user.profile_data ?? {};
+                        return rest;
+                      }
+                    })(),
                   };
                 } else if (person.new_email !== undefined) {
                   return { ...user, email: person.new_email };