Add NullableCondition extension for Condition

[nickolasdeluca]

Introduce a NullableCondition<T> extension on Condition<T>? providing and(Condition<T>) and or(Condition<T>) helpers.

These methods return the provided condition when the receiver is null, or delegate to the existing and/or logic otherwise, simplifying conditional accumulation of query conditions and removing boilerplate null checks. Doc examples included to demonstrate usage when building queries.

Usefull when building queries with multiple condition options that can all be supplied as null.

Example:

Condition<City>? condition;

if (id != null ) condition = condition.and(City_.id.equals(id));
if (name != null && name.isNotEmpty) condition = condition.and(City_.name.contains(name));

query = box.query(condition).build();

I always write this extension on all my projects because it makes it much easier to work with lots of conditions. Since Condition itself is not nullable, I'd always have to kick start it with something like City_.id.greaterThan(0) and then add my conditional conditions (haha).

Providing this extension natively should help everyone, I hope.