UI - Only include custom sourced emails that are present (#24411)

## Addresses #24321 

Note that the "Used by" section includes the Google chrome user and the
custom user that has an associated email, but ignores the custom user
with no email:

<img width="1349" alt="Screenshot 2024-12-04 at 9 37 10 PM"
src="https://github.com/user-attachments/assets/b03b519b-c904-4327-bf76-494a3c80a27b">

- [x] Changes file added for user-visible changes in `changes/`, 
- [x] Manual QA for all new/changed functionality

---------

Co-authored-by: Jacob Shandling <jacob@fleetdm.com>

changes/24321-exclude-custom-source-without-email

@@ -0,0 +1,2 @@
+- Exclude any custom sourced "users" from the host details "used by" display if Fleet doesn't have
+  an email for them.

frontend/interfaces/host.ts

@@ -119,7 +119,10 @@ export const mapDeviceUsersForDisplay = (
         idpUser = d;
         break;
       case "custom":
-        customUser = d;
+        // exclude custom user without email
+        if (d.email) {
+          customUser = d;
+        }
         break;
       default:
         newDeviceMapping.push(getDeviceUserForDisplay(d));

frontend/pages/hosts/details/HostDetailsPage/HostDetailsPage.tsx

@@ -39,7 +39,6 @@ import {
   IQueryKeyQueriesLoadAll,
   ISchedulableQuery,
 } from "interfaces/schedulable_query";
-import { IHostScript } from "interfaces/script";
 
 import {
   normalizeEmptyValues,