SUMPRODUCT treats blank cells as containing "1" rather than "0" - only in first argument: New in 126.96.36.199
Thanks for reporting that.
This was previously reported in Bug 74479 - CALC problem counting weekdays with sumproduct. Only Saturdays are not counted correctly when the selection contains empty cells or no date. SUMPRODUCT() result error with functions or calculations the last parameter.
This is already fixed and patch is available in LO 4.2.1.
*** This bug has been marked as a duplicate of bug 74479 ***