Support for `empty` types is not yet implemented in incremental smt2 decision procedure

[esteffin]

Support for the empty type is not yet implemented in incremental smt2 decision procedure.

This causes a failure with invariant in convert_type_to_smt_sort about unimplemented generation of formulas for typeempty as an example.

This is required for the following test:

• cbmc/havoc_slice_checks/test.desc
• cbmc/memset_null/test.desc