JsonObject's getString() should provide same interface as other getters

`JsonObject`'s getters are inconsistent in the interfaces that they provide.

`getString("thing")` throws an exception if the key is missing, all other getters return null.

As a convenience method, `getString()` should provide the same interface as the other getters, which all seem to model `Map.get()` in returning null when the key is missing.

The obvious workaround is to use `getString("thing", null)`. But `getString("thing")` should not be violating the interface provided by all other methods.

Start at the JsonObject getString("thing") entry point and compare its missing-key behavior with the other JsonObject getters. Confirm how the overload accepting a default value behaves. Done means the no-default getString() call returns null for a missing key, matching the other getters and the Map.get() model.

Written by the indexing model from the issue text.