expand: Fixed "$@" expansion when EXP_FULL is false

The commit 3c06acdac0b1ba0e0acdda513a57ee6e31385dce ([EXPAND]
Split unquoted $@/$* correctly when IFS is set but empty) broke
the case where $@ is in quotes and EXP_FULL is false.

In that case we should still emit IFS as field splitting is not
performed.

Reported-by: Juergen Daubert <jue@jue.li>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>