Empty DO_NADA causes make: exec() failed (No such file or directory)

[Edgar Fuß <ef%math.uni-bonn.de@localhost>]

Just for the record: The change introduced by jperkin@ in mk/bsd.pk.mk 1.2044 
(setting DO_NADA to empty instead of ${TRUE}) makes older/ancient versions 
of make choke myteriously:
	make: exec() failed (No such file or directory)
It took quite a while (and help of a colleague) to spot this was not actually 
some exec system call failing, but make itself.

The Problem seems to be
	...
		@${DO_NADA}
in various places since
	default:
		@
fails on old (b)make versions.

I guess it could be fixed by setting SILENT_DO_NADA to empty/@${DO_NADA}, 
but that may be not be worth the trouble.

I stumbled over this when trying to build 2022Q4 for NetBSD-6 (actually, 
in a chroot on NetBSD-8) for a machine I don't want to update.