efi_loader: SetVariable() deleting variables

APPEND_WRITE with data length zero is allowable according to the UEFI
specification.

The EDK2 interpretation of no access attributes is attributes = 0. As
the UEFI specification is vague in this respect let's stick to EDK2 here.

Fixes: commit 6d2f27c5 ("efi_loader: variable: support APPEND_WRITE")
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>