Fix null pointer dereference in mlir::GetOutermostOpsOfType
Here are the internal errors, @SweetVishnya can you please verify ? Thank you!

Traceback (most recent call last):
  File "/py/absl/testing/parameterized.py", line 320, in bound_param_test
    return test_method(self, *testcase_params)
  File "/tensorflow/python/tpu/google/sparse_core/tpu_embedding_v3_test.py", line 658, in test_single_feature_single_table_backwards_pass_with_csr_input_with_minibatching
    tpu_embedding_v3_test_utils.create_input_data_based_on_hw_requirement(
TypeError: create_input_data_based_on_hw_requirement() missing 1 required positional argument: 'num_minibatches_per_physical_sparse_core' > Here are the internal errors, @SweetVishnya can you please verify ? Thank you!
> 
> Traceback (most recent call last): File "/py/absl/testing/parameterized.py", line 320, in bound_param_test return test_method(self, *testcase_params) File "/tensorflow/python/tpu/google/sparse_core/tpu_embedding_v3_test.py", line 658, in test_single_feature_single_table_backwards_pass_with_csr_input_with_minibatching tpu_embedding_v3_test_utils.create_input_data_based_on_hw_requirement( TypeError: create_input_data_based_on_hw_requirement() missing 1 required positional argument: 'num_minibatches_per_physical_sparse_core'

This seems to be unrelated to the code affected by this PR. I'll try to rebase when I get to PC. @gbaned, rebased on master. I hope the unrelated test crashes will go away. @gbaned, can you, please, rerun internal checks, so, we can continue merging process? > @gbaned, can you, please, rerun internal checks, so, we can continue merging process?

Hi @SweetVishnya  Sorry for the delay. I have triggered the checks. Thank you! > > @gbaned, can you, please, rerun internal checks, so, we can continue merging process?
> 
> Hi @SweetVishnya Sorry for the delay. I have triggered the checks. Thank you!

It seems that internal checks were cancelled: kokoro team removed the label. Maybe, we need additional approval from @mihaimaruseac The kokoro label is removed when the kokoro CI starts, it's just the way the automation works.
The bug was found by Svace static analyzer:

1. v is null
2. v.emitError() dereferences a null pointer

cc @mihaimaruseac