[Storage] arb_existent_kvs_and_nonexistent_keys should create at least one key
[Storage] arb_existent_kvs_and_nonexistent_keys should create at least one key

The output of this function is given to `init_mock_db`, which asserts
that there should be at least one key.

Closes: #5981

aptos-labs/aptos-core,libra/libra,libra/libra,libra/libra,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,libra/libra,aptos-labs/aptos-core,aptos-labs/aptos-core,aptos-labs/aptos-core