where(foo: [1, nil]) becomes "WHERE foo = 1 OR foo IS NULL"

was "WHERE foo IN (1) OR foo IS NULL" before

activerecord/lib/active_record/relation/predicate_builder.rb

@@ -29,8 +29,11 @@ def self.build_from_hash(engine, attributes, default_table)
 
             if values.include?(nil)
               values = values.compact
-              if values.empty?
+              case values.length
+              when 0
                 array_predicates << attribute.eq(nil)
+              when 1
+                array_predicates << attribute.eq(values.first).or(attribute.eq(nil))
               else
                 array_predicates << attribute.in(values).or(attribute.eq(nil))
               end