Fix uniqueness validator with allow_nil: false also for mysql

MySQL's adapter was changing value for case sensitive comparison. We
don't need to do it for `nil`.

Author: drogus

activerecord/lib/active_record/validations/uniqueness.rb

@@ -60,7 +60,7 @@ def build_relation(klass, table, attribute, value) #:nodoc:
           # will use SQL LOWER function before comparison, unless it detects a case insensitive collation
           relation = klass.connection.case_insensitive_comparison(table, attribute, column, value)
         else
-          value    = klass.connection.case_sensitive_modifier(value)
+          value    = klass.connection.case_sensitive_modifier(value) if value
           relation = table[attribute].eq(value)
         end