Avoid NPE for anonymous admin permissions check

This avoids a potential NPE when jenkins.model.Jenkins.getUser(name) ends up
calling hasPermissions for "hudson.model.Hudson.Administer".  I ran into this
on a jenkins instance that had greenballs-plugin installed, and READ access
for anonymous users enabled.

- Verify read permissions before performing anonymous authorization checks
- Make requestURI return null if Stapler.currentRequest is null
- Make currentUriPathEquals return false if requestURI is null