There are entries in the revision table on enwiki where rev_len is NULL. This is rather silly, and also, currently causes the {{PAGESIZE:}} parser function to return unexpected results for affected pages (T20998). We should probably fix them. (Originally reported at https://en.wikipedia.org/wiki/Template_talk:Ifexist_not_redirect#Bug:_Expression_error:_Unexpected_.3E_operator.)
I did some investigating and it looks like this was caused by a bug in 429ae88d (part of ContentHandler branch, merged into master in 329d5b35) and resolved in 4fb4ad91. Thus affected revisions in production should be limited to time between 1.21wmf2 and 1.25wmf2 – 2012-10-15 to 2014-10-10, and the correct rev_len value for them all is simply 0.
This would be trivial to fix with a maintenance script (we even have one, might need tweaks though: populateRevisionLength.php), but since there are no useful indexes, this would require a full table scan and probably take days.