"empty" local pages

[tianon]

Relevant to #84, and as seen in https://github.com/docker-library/repo-info/blob/eeaf3b02d1fc04427f9bb0c1f28bf95cd74707b8/repos/mariadb/local/11.4.md, sometimes glitches mean empty pages.

This is exceptional -- there are some valid cases of it, but we should make them be special cases and have something alerting/warning us in general.

[ramazansancar]

This problem seems to be solved. @tianon

[tianon]

The "empty" pages are, yes, but this issue is not about fixing that, but rather having something warning us if/when it happens again.

we should make them be special cases and have something alerting/warning us in general