[BUGFIX] Handle null values correctly to avoid PHP errors

The new RteHtmlParser->transformTextForPersistence() method
expects a string value. Before this patch the given value
was simply passed without checking its type or casting it.

However, if "null" is handed in (by e.g. a translated record which
expects null), null is now kept.

Resolves: #91749
Releases: master, 10.4
Change-Id: I9db872ca73dcf2bbfc2ac2d0b67d45ca3ffd4c5e