mk/haskell.mk: fix PLIST status detection

Before, running "HS_UPDATE_PLIST=yes bmake update" in wm/xmonad did not
apply the proper substitutions to the generated PLIST file since the
PLIST file was created empty during the GENERATE_PLIST command, and that
empty PLIST file changed the status to "plain" instead of "missing".
Because of that, the HS_INTF and related placeholders were not defined.

The 2 conditions for the status "missing" had to be written in separate
.if clauses because of a bug in bmake that was introduced in 2015 and
will be fixed with the next bmake update.  For further details, see
src/usr.bin/make/unit-tests/cond-short.mk.

(rillig)