Check page_len in ResourceLoaderWikiModule::isKnownEmpty() for 'user' modules

In most cases, we just check whether the pages exist before saying
the module is not empty to avoid generating cached HTML without
the appropriate <script> or <link> tags.

However, for modules in the 'user' group, normal users cannot
delete their personal JavaScript/CSS pages, causing needless
extra requests, even though we know the pages are empty.

ResourceLoader::isKnownEmpty() now checks the page_len field
for modules in the 'user' group to check that there is
some actual content.

Bug: 68488
Change-Id: I0570f62887fd4642fd60367ae0b51d7dc19488ca