Handle model deserialization when output tensor shape is NULL. (tensorflow#22219)

In flatbuffers, vectors default to NULL.

Original change by alanchiao@.

PiperOrigin-RevId: 212506392

Author: suharshs

tensorflow/contrib/lite/model.cc

@@ -240,6 +240,11 @@ TfLiteStatus InterpreterBuilder::BuildLocalIndexToRegistrationMapping() {
 namespace {
 template <class T>
 std::vector<int> FlatBufferIntArrayToVector(T* flat_array) {
+  // Initialize shape of tensors with null shape. Empty vectors are converted
+  // to nullptr for models that are constructed via flatbuffers::Pack.
+  if (flat_array == nullptr) {
+    return {};
+  }
   std::vector<int> ret(flat_array->Length());
   for (int i = 0; i < flat_array->Length(); i++) {
     ret[i] = flat_array->Get(i);