Fix _present / _blank producing always-UNKNOWN clause on non-string columns (#1552)

[ryoya1122]

Closes #1552.

Problem

The built-in _present (and mirrored _blank) predicate produces broken SQL on every non-string column:

Product.ransack(stock_present: true).result.to_sql
# => SELECT "products".*
#    FROM "products"
#    WHERE ("products"."stock" IS NOT NULL
#       AND "products"."stock" != NULL)     ← always UNKNOWN

The trailing != NULL is never true, so stock_present: true returns 0 rows even when every row has a non-null stock. The reporter in #1552 hit the same shape on a Postgres string column with a unique partial index, but it reproduces just as cleanly on any boolean / integer / decimal / datetime column.

String / text columns are unaffected — they correctly produce the documented IS NOT NULL AND != '' form.

Cause

The present / blank predicate definitions in lib/ransack/constants.rb ship a column-type-agnostic formatter:

formatter: proc { |v| [nil, ''.freeze].freeze }

Condition#format_predicate feeds those values into Arel's NOT_EQ_ALL / EQ_ANY. For non-string columns, ActiveRecord casts '' to the column type, which is NULL, leaving the always-UNKNOWN comparison.

Fix

Condition#format_predicate now strips the empty-string entry from arel_values when the attribute is not a string-like column (:string, :text, :citext). The two-clause form is preserved on string / text columns; non-string columns reduce to IS NOT NULL (or IS NULL for _blank) — which matches what Object#present? means for those types in Rails.

# After:
Product.ransack(stock_present: true).result.to_sql
# => SELECT "products".* FROM "products" WHERE ("products"."stock" IS NOT NULL)

Compatibility

Column type	Predicate	Before	After
:string / :text / :citext	_present (true)	IS NOT NULL AND != ''	IS NOT NULL AND != '' ✓ unchanged
:string / :text / :citext	_blank (true)	IS NULL OR = ''	IS NULL OR = '' ✓ unchanged
:integer / :decimal / :boolean / :datetime / etc.	_present (true)	IS NOT NULL AND != NULL ❌	IS NOT NULL ✓ fixed
:integer / :decimal / :boolean / :datetime / etc.	_blank (true)	IS NULL OR = NULL ❌	IS NULL ✓ fixed
unknown / nil type (e.g. ransackers without explicit type)	_present / _blank	two-clause form	two-clause form ✓ unchanged (treated as string-like, conservative default)

The condition for stripping is intentionally narrow:

if arel_values.is_a?(Array) && arel_values.include?('') && !string_like_attribute?(attribute)
  arel_values = arel_values.reject { |v| v == '' }
end

• Only fires when the formatter actually emitted '' — custom predicates that don't use that envelope are untouched.
• Only fires when the attribute is provably non-string-like — unknown types keep the existing behaviour.

Tests

Four new specs in spec/ransack/predicate_spec.rb, beside the existing string-column ones:

• _present / _blank on salary (integer) emits only IS [NOT] NULL, never != NULL or = NULL
• existing name (string) tests continue to pass, verifying the two-clause form is preserved

Full suite: 516 examples, 0 failures, 1 pending on v5.0.0 + this branch (SQLite, ActiveRecord 7.2.3.1, Ruby 3.4.9).

Targets v5.0.0 per #1640.