Arm backend: Fix mypy warning in runner_utils.py

[perheld]

Add a return None if elf_path is not set.

Signed-off-by: per.held@arm.com
Change-Id: Ib03c31cb2a29ce4635d3294121037904012d2fde

cc @freddan80 @per @zingo @oscarandersson8218 @digantdesai

[pytorch-bot]

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✅ You can merge normally! (6 Unrelated Failures)

As of commit 88ddd5d with merge base d07a49a ():

BROKEN TRUNK - The following jobs failed but were present on the merge base:

👉 Rebase onto the `viable/strict` branch to avoid these failures

• pull / test-binary-size-linux-gcc / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / test-openvino-linux / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / test-setup-linux-gcc / linux-job (gh) (trunk failure)
  ##[error]The operation was canceled.
• pull / unittest / windows / windows-job (gh) (trunk failure)
  backends/xnnpack/test/ops/test_conv1d.py::TestConv1d::test_qs8_conv1d_batchnorm_seq
• pull / unittest-editable / windows / windows-job (gh) (trunk failure)
  backends/xnnpack/test/ops/test_conv1d.py::TestConv1d::test_qs8_conv1d_batchnorm_seq
• trunk / unittest-release / windows / windows-job (gh) (trunk failure)
  backends/xnnpack/test/ops/test_conv1d.py::TestConv1d::test_qs8_conv1d_batchnorm_seq

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