fix crash during serialization due to NULL string
fix saving, loading and json serializatin of non-persistent forms (0xFF* form id). thanks to verteiron for insight
add lost changes
Merge branch 'master' into develop

Conflicts:
	JContainers/src/collections.tests.hpp
	JContainers/src/json_handling.h
	JContainers/src/serialization.cpp
move some common form operations like mod index extraction and etc into separate function