ToString() from a StringTable entry now returns <MISSING STRING TABLE ENTRY>

This is a regression from 5.3 that has been fixed in Main (32476819, for 5.5), but needs merging into 5.4.

1. Create a String Table and add an entry to it.
2. Reference that entry as an FText and call ToString() on it.
3. Observe that it returned "<MISSING STRING TABLE ENTRY>" rather than the source string of the entry.