Fix index_exists? to handle empty columns argument

Author: fatkodima

activerecord/lib/active_record/connection_adapters/abstract/schema_definitions.rb

@@ -48,7 +48,7 @@ def column_options
       end
 
       def defined_for?(columns = nil, name: nil, unique: nil, valid: nil, **options)
-        columns = options[:column] if columns.nil?
+        columns = options[:column] if columns.blank?
         (columns.nil? || Array(self.columns) == Array(columns).map(&:to_s)) &&
           (name.nil? || self.name == name.to_s) &&
           (unique.nil? || self.unique == unique) &&

activerecord/test/cases/migration/index_test.rb

@@ -172,6 +172,7 @@ def test_index_exists_on_multiple_columns
       def test_index_exists_with_custom_name_checks_columns
         connection.add_index :testings, [:foo, :bar], name: "my_index"
         assert connection.index_exists?(:testings, [:foo, :bar], name: "my_index")
+        assert connection.index_exists?(:testings, [], name: "my_index")
         assert_not connection.index_exists?(:testings, [:foo], name: "my_index")
       end