Adjust remaining uses of `NILP (HASH_HASH)`.

Adjust remaining uses of `NILP (HASH_HASH)`.

* src/json.c (lisp_to_json_toplevel_1):
* src/pdumper.c (dump_hash_table_stable_p, hash_table_contents):
* src/print.c (print, print_vectorlike):
* src/minibuf.c (Ftry_completion, Fall_completions, Ftest_completion):
Use `EQ (HASH_KEY, Qunbound)` instead of `NILP (HASH_HASH)`.