Create this filter, on an active wiki:
action == "edit" & page_id !== 0 & length(added_lines + removed_lines) == 0
The actual result was 100 hits in only 5 hours.
The expected result is no matches; null edits shouldn't even be tested against any filter.
Saving a null edit in the 2017 wikitext editor causes this, reliably. But most of those hits can't be explained by that; logged out editors don't even have that option available. Null edits in the old wikitext editor, VisualEditor, or the mobile editor don't seem to cause this.
Some other hits might be explained by two editors saving the same content at almost exactly the same time; for example we have multiple bots that fix double-redirects.
But whatever the cause, getEditVars() should have bailed out before anything was tested against the filter.