APISite.logevents: Allow empty list as the namespace parameter
APISite.logevents: Allow empty list as the namespace parameter

This fixes a regression introduced in ecb4bd67393c158ab9e9406d567dea4208ffd9a8
which was causing test_logpages_dump to fail because the default value for
`namespace` parameter is `[]`.

Change-Id: I9ae4cac160702b12b49ff258e02b0acf5cdbfbad

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