[tflite] Test for `kTfLiteOptionalTensor` in `GetInput`.

`GetInput`, `GetVariableInput` and `GetOutput` all fail to check for the case where `node->inputs->data[index]` is the special `kTfLiteOptionalTensor` value (-1) which then causes `context->tensors[node->inputs->data[index]]` to read from invalid memory location.

This fix makes `GetInput` and related return `nullptr` in those cases, asking the caller to check for `nullptr`. This is better than having `GetOptionalInputTensor` and `GetOptionalOutputTensor` (does not exist but could be added) as using the patched `GetInput` in error would be caught by a sanitizer test in the default optimized build (due to the `-fsanitize=null` option).

PiperOrigin-RevId: 332512190
Change-Id: Iabca54da2f2de02b6ece3c38b54f76d4277d689e