[PP-1] Change clause in InOperator::acceptExposedInput() for 'reduce' to test for TRUE instead of !empty()

Problem/Motivation

Split from #3132725: "Limit list to selected items" on exposed filters does not filter and postponed on that being committed.

The underlying logic for the 'Limit list to selected options' is controlled here:

InOperator::acceptExposedInput() {
...
  if (empty($this->options['expose']['multiple']) && empty($this->options['expose']['required']) && !empty($this->options['expose']['limit'])) {
  }
...
}

That last clause is broken, because the actual setting is called 'reduce' not 'limit'. That's what #3132725 fixes.

However, there's a question as to why we're testing it via !empty() instead of $this->options['expose']['reduce'] === TRUE.

This issue is to explore that and fix the logic.

Steps to reproduce

Proposed resolution

Remaining tasks

User interface changes

Introduced terminology

API changes

Data model changes

Release notes snippet