test(C): exercise reads on an empty DB in delete_all_keys

[ALH477]

Strengthens the existing delete_all_keys test (C reference impl).

Today it deletes both keys then re-inserts, but never reads the now-empty database — and a read is exactly what dereferences db->root. This adds, right after the two deletes:

size_t vsize = 0;
ASSERT_NULL(streamdb_get(db, (const unsigned char*)"a", 1, &vsize));
StreamDBResult* empty = streamdb_suffix_search(db, (const unsigned char*)"a", 1);
ASSERT_NULL(empty);
streamdb_free_results(empty);

This locks in the "root is never NULL" guard in streamdb_delete (which re-seeds an empty root after the prune frees it). An earlier reverse-trie variant lacking that guard segfaulted on exactly this path (a get/suffix_search after the last key is deleted) — heap-layout-sensitive, so it looked intermittent. Current main already handles it; this test makes that explicit and guards against regressions.

Test-only change. Full suite passes locally (24/24) under the repo's -Werror -pedantic build:

delete_all_keys                                    [PASS]
Results: 24 passed, 0 failed

Context: opened from #4.

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