Site.page_restrictions(): Do not raise NoPage
Site.page_restrictions(): Do not raise NoPage

Nonexistent pages can be protected, so treat all pages the same.

Bug: T214286
Change-Id: I2bcb85a294ea3ce9709cc9d7700abea87e3f841c

wikimedia/pywikibot-core,wikimedia/pywikibot-core,PersianWikipedia/pywikibot-core