fix: make sure CASE WHEN pick first true branch when WHEN clause is true

[haohuaijin]

Which issue does this PR close?

Closes #8475

Rationale for this change

In our current implementation, when WHEN clause is true, we will always pick the last true branch.
Because when WHEN is true(WHEN true or the expression can simplified to WHEN true), we don't set when_value to false (even though it already has the result)in the below code, so we always update the current_value.
https://github.com/apache/arrow-datafusion/blob/2765fee4dd7518a4dc39244d1b495329b9771be1/datafusion/physical-expr/src/expressions/case.rs#L194-L208

What changes are included in this PR?

Are these changes tested?

Are there any user-facing changes?

[alamb]

Looks good to me -- thank you @haohuaijin

[alamb]

cc @yjshen in case you have afew moments to review

[alamb]

Thanks everyone!