TypeHandler SerDe is backwards incompat with older versions of FM

While working on OODT-139 and testing out with a production system at JPL running the old internal JPL OODT code, I noticed that prior versions of the file manager (pre-Apache) are incompatible with the existing Apache code due to a simple check that isn't being done in the XmlRpcStructFactory SerDe inside of the file manager with respect to the type handlers (introduced in the JPL internal version of the file manager version 1.8.0 and beyond). It would be useful to projects here at JPL (and outside of it) that use our software pre-Apache and who want to upgrade or get on the Apache version to have a fix that checks to make sure that type handlers actually exist before SerDe'ing them. In fact, this is a blocker to using PCS Health Monitor and the newer PCS code as a whole with older legacy systems because the FM SerDE breaks.

I'll commit a simple fix for this shortly.