When the setting "persistUndefinedAsNull" is true,
the model will use `null` instead of `undefined` in
all property values.
- Known optional model properties are set to `null` when no value
was provided.
- When setting model properties, `undefined` is always converted
to `null`. This applies to both known (model-defined) properties
and additional (custom, dynamic) properties.
- The instance method `toObject()` converts `undefined` to `null` too.
Because `toJSON()` calls `toObject()` under the hood, the change
applies to `toJSON()` too.
For queries, this is undesirable. It also affects
loopback-connector-mongodb's ability to perform nested queries (it
expects plain objects to work correctly, and to allow $elemMatch for
example).
Miroslav Bajtoš
Raymond Feng
Pradnya Baviskar
Fabien Franzen
arlaneenalra
Ritchie Martori
Anatoliy Chakkaev