NULL havoc avoidance lacks struct and array support

[ArenBabikian]

CBMC version: 5.29.0
Operating system: Ubuntu20
Exact command line resulting in the issue: See below
What behaviour did you expect: n/a
What happened instead: n/a

---

This issue pertains to the implementation of a new feature for CBCM that avoids havocing NULL assignable symbols (see #6077).

Specifically, the changes proposed in #6077 only support pointer objects.
This issue indicates the lack of support for structs and arrays.