BaseTools/Python: Allow HOST_APPLICATION to use NULL libraries

BaseTools/Python: Allow HOST_APPLICATION to use NULL libraries

REF: https://bugzilla.tianocore.org/show_bug.cgi?id=2797

Update HOST_APPLICATION module type to use NULL library instances.

Cc: Bob Feng <bob.c.feng@intel.com>
Cc: Liming Gao <liming.gao@intel.com>
Cc: Sean Brogan <sean.brogan@microsoft.com>
Cc: Bret Barkelew <Bret.Barkelew@microsoft.com>
Cc: Jiewen Yao <jiewen.yao@intel.com>
Signed-off-by: Michael D Kinney <michael.d.kinney@intel.com>
Reviewed-by: Bob Feng <bob.c.feng@intel.com>
Reviewed-by: Sean Brogan <sean.brogan@microsoft.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>