Unfortunately, Maddock Emerson has reported in the Community forum that he has seen the file-replaced-by-NULs in Notepad++ v7.7.1, when he allowed the OS to reboot due to OS update.

This means that abc6bc1 (and @pnedev's changes that led up to that commit) appears to not have been sufficient to eliminate all the possible conditions that can cause the NUL file. If @pnedev is able to join into the forum discussion, as the one who made the fixes for v7.5.9, he might be able to elicit more info from Maddock Emerson about the problem, and see what's different about this circumstance than the ones that were fixed.