Fix: blank filter values no longer activate their filter (1.2.1)

[bunnrf]

Why

1.2.0 changed Filtrator#active_filters so a filter activates whenever its param key is present, regardless of the value. This means a blank value (nil, "", []) now activates the filter — which the pre-1.2 code deliberately skipped.

In practice that turns an empty filter into a real SQL predicate:

• WHERE col = '' — harmless on some string columns, but a 500 (PG::InvalidTextRepresentation) on integer / uuid columns.
• WHERE col IN () — unintended narrowing to zero rows.
• undefined scopes invoked with a blank arg — NoMethodError.

Blank filter values are extremely common (a UI sends filters[status]= for an unselected dropdown), so consumers on 1.2.0 see production errors on ordinary requests.

What

Restore the pre-1.2 rule that a blank value leaves its filter inactive, while keeping the 1.2.0 fix that a literal boolean false is a meaningful, active value (#58).

active_filters now activates a filter only when:

value == false || value.present?

so false still filters, but nil / "" / [] no longer do.

Testing

• Added a Filtrator test asserting blank values ("", nil, []) leave the filter inactive and return the full collection. This fails on 1.2.0 and passes here.
• The existing "filters by boolean field even when the value is false" test still passes, guarding the Fix filter by boolean field #58 behavior.

Bumps the version to 1.2.1 and adds a CHANGELOG entry.