Fix round-tripping plPythonFileMod through HSPlasma.

This fixes an issue in which PythonFileMods with string parameters would
be shattered by round-tripping through HSPlasma. The spurious null
terminator was being accumulated into the parameter, causing two nulls
to be serialized. This caused a null byte to be passed to CWE's Python,
resulting in a difficult to debug scenario where we had a seemingly
valid string object in Python but the validation in `PyArg_ParseTuple`
failed.

core/PRP/Modifier/plPythonFileMod.cpp

@@ -82,10 +82,11 @@ void plPythonParameter::read(hsStream* S, plResManager* mgr)
     case kSubtitle:
         size = S->readInt();
         if (size == 0) {
-            fStrValue = "";
+            fStrValue = ST::null;
             return;
         }
-        fStrValue = S->readStr(size);
+        fStrValue = S->readStr(size - 1);
+        S->readByte();
         return;
     case kNone:
         return;