surfacecontrol: mark mCalls Nullable & fix missing null checks

- Check for null properly for debug function to prevent runtime restarts
- NPE can occur when sLogAllTxCallsOnApply is reset. In operations
  involving two transactions (e.g. merge), we only check null for mCalls
  but not other.mCalls. This commit fixes this case.

Test: manual
Bug: 266978825
Flag: EXEMPT, bug fix
Change-Id: I5c480a410cf91b04c44deb00b12eb169216f899e