Use of uninitized prev variable

The CI job #52072 failed showing a merge output with the first item skipped.

This failure is caused by the use of uninitialized memory prev_b due to the input array a being empty. This only occurs with the DROP_IN_INPUT duplicate strategy. The same issue can occur if prev is uninitialized due to an empty input array a with the duplicate strategy DROP. In both cases the prev or prev_b value has the same value as the first item in array b and is thus skipped.