Fix unreachable code warning (#25215)
### Description
<!-- Describe your changes. -->
Debug Windows build fails with unreachable code warning due to change
added in #25161.
Use an `else` to avoid the warning.
```
\onnxruntime\test\contrib_ops\matmul_4bits_test.cc(559,1): error C2220: the following warning is treated as an error [\build\Windows.vs22\Debug\onnxruntime_test_all.v cxproj]
\onnxruntime\test\contrib_ops\matmul_4bits_test.cc(559,1): warning C4702: unreachable code [\build\Windows.vs22\Debug\onnxruntime_test_all.vcxproj]
\onnxruntime\test\contrib_ops\matmul_4bits_test.cc(561,1): warning C4702: unreachable code [\build\Windows.vs22\Debug\onnxruntime_test_all.vcxproj]
...
```
### Motivation and Context
<!-- - Why is this change required? What problem does it solve?
- If it fixes an open issue, please link to the issue here. -->
