[infra] Remove "null" check from UBSan options. (#674)
[infra] Remove "null" check from UBSan options. (#674)

There is not much value for us from this particular check:
- UBSan's "null" reports leading to real crashes are reported with ASan as well.
- Potential null references are not real crashes and might be considered
  as false positives, though it seems to be UB as per the standard.
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