import: Don't assume user_profile_id attribute is set on emojis.

The s3 import code path made a hard assumption about `user_profile_id`
being set (we'd already fixed this in the local uploads code path).

Ideally, it should be, and I've opened #10268 for fixing that, but for
now this is how it needs to work.

zerver/lib/import_realm.py

@@ -505,6 +505,7 @@ def import_uploads_s3(bucket_name: str, import_dir: Path, processing_avatars: bo
 
     re_map_foreign_keys_internal(records, 'records', 'realm_id', related_table="realm",
                                  id_field=True)
+    if not processing_emojis:
         re_map_foreign_keys_internal(records, 'records', 'user_profile_id',
                                      related_table="user_profile", id_field=True)
     for record in records:
@@ -534,6 +535,9 @@ def import_uploads_s3(bucket_name: str, import_dir: Path, processing_avatars: bo
             key.key = s3_file_name
             path_maps['attachment_path'][record['s3_path']] = s3_file_name
 
+        # Exported custom emoji from tools like Slack don't have
+        # the data for what user uploaded them in `user_profile_id`.
+        if not processing_emojis:
             user_profile_id = int(record['user_profile_id'])
             # Support email gateway bot and other cross-realm messages
             if user_profile_id in id_maps["user_profile"]:
@@ -541,8 +545,9 @@ def import_uploads_s3(bucket_name: str, import_dir: Path, processing_avatars: bo
                 user_profile_id = id_maps["user_profile"][user_profile_id]
             user_profile = get_user_profile_by_id(user_profile_id)
             key.set_metadata("user_profile_id", str(user_profile.id))
-        key.set_metadata("realm_id", str(user_profile.realm_id))
+
         key.set_metadata("orig_last_modified", record['last_modified'])
+        key.set_metadata("realm_id", str(record['realm_id']))
 
         headers = {'Content-Type': record['content_type']}