base.bbclass: Work even when there's no PNBLACKLIST entries

base.bbclass: Work even when there's no PNBLACKLIST entries

It turns out that the result of getVarFlags is not a list, it's
a dict. So "getVarFlags(...) or []" does not reliably produce
something with a .items. This escaped detection because our
local build environment never ends up running builds without
PNBLACKLIST entries.

(From OE-Core rev: 491df239170dd20f3e91df6bc1ead2945a78e075)

Signed-off-by: Peter Seebach <peter.seebach@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>