test: add json test for parsing from map
test: add json test for parsing from map

Our JSON legacy helper functions for parsing documents to/from
string maps are indirectly tested by several unit tests, e.g.
caching_options_test.cc. They however lacked one corner case
detected only by dtest - parsing an empty map from a null JSON document.
This case is hereby added in order to prevent future regressions.

Message-Id: <49d70210bdb864f6f81524ab2ba614561ff6fa12@scylladb.com>

avikivity/scylla,avikivity/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla