Check whether the bias tensor is `nullptr` before accessing the type. (#2566)

Prevents program crash due to null pointer dereference.

BUG=none

tensorflow/lite/micro/kernels/transpose_conv.cc

diff --git a/tensorflow/lite/micro/kernels/transpose_conv.cc b/tensorflow/lite/micro/kernels/transpose_conv.cc
index cd61660..ea0efae 100644
--- a/tensorflow/lite/micro/kernels/transpose_conv.cc
+++ b/tensorflow/lite/micro/kernels/transpose_conv.cc
@@ -15,6 +15,9 @@
 
 #include "tensorflow/lite/kernels/internal/reference/transpose_conv.h"
 
+#include <cstddef>
+#include <cstdint>
+
 #include "tensorflow/lite/c/builtin_op_data.h"
 #include "tensorflow/lite/c/common.h"
 #include "tensorflow/lite/kernels/internal/common.h"
@@ -48,8 +51,9 @@
   // A scratch buffer is required for quantized implementations.
   int scratch_buffer_index;
 
-  // TODO(b/192090531): Remove this once all 8x16 transpose conv models use
-  // 64-bit biases.
+  // Index to the converted 64-bit bias buffer from 16-bit bias. This is
+  // required to handle 16x8 transpose convolutions where a 16-bit bias is
+  // provided, whereas the kernel expects 64-bit biases.
   int bias_converted_buffer_index;
 
   // Multiplier and shift arrays are required for the int8 implementation.
@@ -123,7 +127,9 @@
     if (input->type == kTfLiteInt16) {
       TFLITE_DCHECK(filter->type == kTfLiteInt8);
       TFLITE_DCHECK(output->type == kTfLiteInt16);
-      if (bias->type == kTfLiteInt16) {
+      // Handle the case where the bias is 16 bits for 16x8 transpose
+      // convolution where the kernel actually expects 64-bit biases.
+      if (bias != nullptr && bias->type == kTfLiteInt16) {
         TFLITE_DCHECK(
             context->RequestScratchBufferInArena(
                 context, GetTensorShape(bias).FlatSize() * sizeof(std::int64_t),
@@ -299,12 +305,10 @@
       break;
     }
     case kTfLiteInt16: {
-      std::int64_t* scratch_buffer = static_cast<int64_t*>(
+      auto* scratch_buffer = static_cast<int64_t*>(
           context->GetScratchBuffer(context, data.scratch_buffer_index));
-      // TODO(b/192090531): Remove this once all 8x16 transpose conv models use
-      // 64-bit biases.
       if (bias != nullptr && bias->type == kTfLiteInt16) {
-        std::int64_t* bias_converted_buffer =
+        auto* bias_converted_buffer =
             static_cast<int64_t*>(context->GetScratchBuffer(
                 context, data.bias_converted_buffer_index));
         for (int i = 0; i < tflite::micro::GetTensorShape(bias).FlatSize();