Splice in a patch to remove Phriction content rows with no document

The unique key on <documentPHID, version> may fail to apply if any content
rows don't have a valid document. This is rare, but we have some old random
garbage rows on "secure.phabricator.com" which prevent the next patch from
applying. Just toss these rows, they're junk.

resources/sql/autopatches/20180828.phriction.07.c.documentuniq.sql

@@ -0,0 +1 @@
+DELETE FROM phriction_content WHERE documentPHID = '';