Avoid deprecated symbol_exprt default construction in unit test [blocks: #3768]

[tautschnig]

We don't care about either the type or the name in this test, so just use
symbol_exprt::typeless(irep_idt()).

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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 2df4578).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/99837013