Akregator feed plugin: Do not assert for a valid but relative URL

For example, "konqueror --part khtml foo".  This passes QUrl("foo"),
a valid but relative URL with no scheme, to KonqFeedIcon::feedFound().
This then asserts when calling KProtocolInfo::protocolClass() for the
blank scheme.

Note that the test case only works with KHTML, and will not reach this
point (an assert will have already happened within KIO) unless the fix
in frameworks/kio!264 is
implemented.

CCBUG:393496