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, 2 insertions, 2 deletions

diff --git a/activerecord/lib/active_record/validations/uniqueness.rb b/activerecord/lib/active_record/validations/uniqueness.rb
index 2e2ea8c42b..2e2bcec922 100644
--- a/activerecord/lib/active_record/validations/uniqueness.rb
+++ b/activerecord/lib/active_record/validations/uniqueness.rb
@@ -54,7 +54,7 @@ module ActiveRecord
 
       def build_relation(klass, table, attribute, value) #:nodoc:
         column = klass.columns_hash[attribute.to_s]
-        value = column.limit ? value.to_s.mb_chars[0, column.limit] : value.to_s if column.text?
+        value = column.limit ? value.to_s.mb_chars[0, column.limit] : value.to_s if value && column.text?
 
         if !options[:case_sensitive] && value && column.text?
           # will use SQL LOWER function before comparison, unless it detects a case insensitive collation
@@ -81,7 +81,7 @@ module ActiveRecord
       #
       #   class Person < ActiveRecord::Base
       #     validates_uniqueness_of :user_name, :scope => :account_id
-      #   end 
+      #   end
       #
       # Or even multiple scope parameters. For example, making sure that a teacher can only be on the schedule once
       # per semester for a particular class.