[DATABASE] Disable 'NULL' strings evaluation as SQL NULLs

Use $object->sqlValue('NULL') (identical to DataObject_Cast'ing) instead and
fix related issues like (email|sms)settings considering these NULLs as a
false positive for the E-Mail address still being set when it's been removed.

There could also be security implications to the now-disabled approach of
considering 'NULL' strings as SQL NULLs.

scripts/upgrade.php

@@ -62,6 +62,7 @@ function main()
 
         fixupNoticeConversation();
         initConversation();
+        fixupUserBadNulls();
         fixupGroupURI();
         if ($iterate_files) {
             printfnq("Running file iterations:\n");
@@ -121,6 +122,26 @@ function updateSchemaPlugins()
     printfnq("DONE.\n");
 }
 
+function fixupUserBadNulls(): void
+{
+    printfnq("Ensuring all users have no empty strings for NULLs...");
+
+    foreach (['email', 'incomingemail', 'sms', 'smsemail'] as $col) {
+        $user = new User();
+        $user->whereAdd("{$col} = ''");
+
+        if ($user->find()) {
+            while ($user->fetch()) {
+                $sql = "UPDATE {$user->escapedTableName()} SET {$col} = NULL "
+                     . "WHERE id = {$user->id}";
+                $user->query($sql);
+            }
+        }
+    }
+
+    printfnq("DONE.\n");
+}
+
 function fixupNoticeConversation()
 {
     printfnq("Ensuring all notices have a conversation ID...");