Squash `wp export` notice about `skip_comments`
Squash `wp export` notice about `skip_comments`

2ad7b8e21fa introduced a PHP notice because `$this->filters['skip_comments']` is checked, but it won't exist if the flag isn't set.
trepmal/wp-cli,2ndkauboy/wp-cli,gitlost/wp-cli,2ndkauboy/wp-cli,aaemnnosttv/wp-cli,miya0001/wp-cli,trepmal/wp-cli,gitlost/wp-cli,GaryJones/wp-cli,aaemnnosttv/wp-cli,gedex/wp-cli,rodrigoprimo/wp-cli,miya0001/wp-cli,gedex/wp-cli,GaryJones/wp-cli,nyordanov/wp-cli,miya0001/wp-cli,rodrigoprimo/wp-cli,gedex/wp-cli,gitlost/wp-cli,trepmal/wp-cli,GaryJones/wp-cli,nyordanov/wp-cli,nyordanov/wp-cli,rodrigoprimo/wp-cli,aaemnnosttv/wp-cli,2ndkauboy/wp-cli