bslalg_scalardestructionprimitives.t: Fix -Wunused warning.

Fixes:

```
../groups/bsl/bslalg/bslalg_scalardestructionprimitives.t.cpp: In function ‘int main(int, char**)’:
../groups/bsl/bslalg/bslalg_scalardestructionprimitives.t.cpp:757: warning: unused variable ‘null’ [-Wunused-variable]
```

The two variables are only referenced in safe-mode asserts, so add `BSLS_ASSERT_SAFE_IS_ACTIVE` check.