CASSANDRA-21505: CAS EQ/NEQ incorrectly distinguishes null from meaningless empty bytes for fixed-length types due to missing sanitization

[minal-kyada]

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[dcapwell]

this patch doesn't really make sense to me.

CAS IF with equality operators are the only place we need to special case where byte[0] != null, as the rest of the code (which goes through this code path) require byte[0] == null (we are only talking about meaningless emptyness).

So to fix CAS IF we need to break the rest of the CQL surface?