This is a good question, because in fact, the pam_unix authentication module and corresponding helper program chkpwd are very unclear on how they handle things. If you do look at the pam_unix documentation (
here, you'll find it has something like "broken_shadow", which allows the module to "succeed" if something in the shadow file is not right, such as an empty expiration time.
However, it seems like it worked even with "broken_shadow" disabled and an empty expiration time. The only way to know for sure is to look at the source code of the modules.