Helper for use in combination with
.:? to provide default
values for optional JSON object fields.
This combinator is most useful if the key and value can be absent from
an object without affecting its validity and we know a default value
to assign in that case. If the key and value are mandatory, use
.: instead.
Example usage:
v1 <- o .:? "opt_field_with_dfl" .!= "default_val"
v2 <- o .: "mandatory_field"
v3 <- o .:? "opt_field2"