Simplify select_one method.

The select_all method always returns ActiveRecord::Result instance,
and the ActiveRecord::Result includes Enumerable mixin.
If #empty?, #first method returns nil. Thus we can simplify this method.

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

@@ -26,8 +26,7 @@ def select_all(arel, name = nil, binds = [])
       # Returns a record hash with the column names as keys and column values
       # as values.
       def select_one(arel, name = nil, binds = [])
-        result = select_all(arel, name, binds)
-        result.first if result
+        select_all(arel, name, binds).first
       end
 
       # Returns a single value from a record