[tflite] Ensure input tensors don't have `nullptr` buffers.

A crafted TFLite model can force a node to have as input a tensor backed by a `nullptr` buffer. That is, by carefully changing the buffer index in the flatbuffer serialization, we can force the TFLite interpreter to consider a read-only tensor to be a read-write one and assume that there is an operator that has this tensor as output, writing to it and allocating memory before the tensor is used as input. If this does not happen, we get memory corruption.

PiperOrigin-RevId: 332524692
Change-Id: I57ef175152a29020af9ab041dc959e5631dce40f