Removed a lot of now-unnecessary checks from node RNA. The typeinfo pointers in bNodeTree, bNode and bNodeSocket are now always pointing to valid type structs. If a tree, node or socket has an unknown type at load time it will be pointed to a dummy "UndefinedType" struct instead, which has default settings and allows for nice debugging. Checking if node->typeinfo etc. is NULL is not necessary any more, makes code a lot easier and smaller.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@55984 954f8c5b-7b00-dc11-b283-0030488c597c