Fix parsing of files with targetNames:null
Fix parsing of files with targetNames:null

targetNames isn't normative and as such can technically be something
other than an array. tinygltf seems to use targetNames:null in some
cases to designate an empty target array; cgltf previously would refuse
to parse files like this and now it discards non-array targetNames
values instead.

jkuhlmann/cgltf,jkuhlmann/cgltf,jkuhlmann/cgltf