Fix FixedString Primary Key Comparison Logic in ClickHouse

We already use padded comparison between String and FixedString (https://github.com/ClickHouse/ClickHouse/blob/master/src/Functions/FunctionsComparison.h#L333). This PR applies the same logic to field comparison which corrects the usage of FixedString as primary keys. This fixes #14908. #15033 (Amos Bird).