Fix `validates_uniqueness_off :field, :allow_nil => false`

Closes (#5853)

Uniqueness validator was not properly checking if there are any existing
records, when value was `nil` and column was text type. `nil` was
converted to string, which resulted in queries looking like:

```sql
SELECT 1 FROM "posts" WHERE "posts"."title" = '' LIMIT 1
```

instead of

```sql
SELECT 1 FROM "posts" WHERE "posts"."title" IS NULL LIMIT 1
```

1 files changed, 11 insertions, 0 deletions

diff --git a/activerecord/test/cases/validations/uniqueness_validation_test.rb b/activerecord/test/cases/validations/uniqueness_validation_test.rb
index 0f1b3667cc..8708117129 100644
--- a/activerecord/test/cases/validations/uniqueness_validation_test.rb
+++ b/activerecord/test/cases/validations/uniqueness_validation_test.rb
@@ -293,4 +293,15 @@ class UniquenessValidationTest < ActiveRecord::TestCase
     assert w6.errors[:city].any?, "Should have errors for city"
     assert_equal ["has already been taken"], w6.errors[:city], "Should have uniqueness message for city"
   end
+
+  def test_allow_nil_is_false
+    Topic.validates_uniqueness_of(:title, :allow_nil => false)
+    Topic.destroy_all
+
+    Topic.create!("title" => nil)
+    topic = Topic.new("title" => nil)
+    assert !topic.valid?, "topic should not be valid"
+    assert topic.errors[:title].any?, "Should have errors for title"
+    assert_equal ["has already been taken"], topic.errors[:title], "Should have uniqueness message for title"
+  end
 end