Adjust case_when DivideByZeroProtection benchmark so that "percentage of zeroes" corresponds to "number of times protection is needed"

[pepijnve]

Which issue does this PR close?

• Related to Potential optimization for CASE WHEN for protecting against divide by zero #11570.

Rationale for this change

The case_when microbenchmark that covers the pattern CASE WHEN d != 0 THEN n / d ELSE NULL END pattern is parameterised over the percentage of zeroes in the d column. The benchmark uses the condition d > 0 rather than d != 0 though which is a bit misleading. In the '0% zeroes' run one would expect the else branch to never be taken, but because slightly less than 50% of the d values is negative, it's still taken 50% of the time.

This PR adjust the benchmark to use d != 0 instead.

What changes are included in this PR?

• Adjust the divide by zero benchmark to use d != 0 as condition
• Remove the duplicate benchmark, the div-by-zero variant is sufficient to compare changes across branches
• Add a couple of SLTs to cover the CASE pattern

Are these changes tested?

Manual testing

Are there any user-facing changes?

No