Compact order values

What does this MR do and why?

• Compact order_values of a scope in SimpleOrderBuilder class.

Because of default_scope used in some models, SimpleOrderBuilder cannot properly pick up order values.

Example: when a model has a default scope reorder(nil) does not actually remove nil.

> Event.reorder(nil).order_values
=> [nil]

Related to #323730 (closed).

MR acceptance checklist

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