When filter value is falsey, irrelevant warnings written to logs

Problem/Motivation

When a filter criterion is NULL, zero, or the empty string, FilterBuilder logs a warning for "Invalid condition." Let's say that we have a field on nodes called "field_archived", and we want to only return nodes that have a value of zero or NULL on this field. Because the FilterBuilder is validating the condition by checking for a non-falsey value, it will incorrectly flag this condition as invalid (even though the condition will still be applied).

Steps to reproduce

1. Create a checkbox field called field_archived on a content-type. The checkbox should be backed by a value of '1'.
2. Configure a search to filter on this field, returning only nodes in which this checkbox is unchecked.
3. With dblog enabled, run the search.
4. Observe that dblog has logged a message saying either Invalid condition field_archived = '0' or Invalid condition field_archived = NULL

Proposed resolution

In FilterBuilder::buildFilters(), do not check $condition->getValue() for falsiness when validating the condition.

Remaining tasks

None.

User interface changes

None.

API changes

None.

Data model changes

None.