Substitution without values and L:0 doesn't add \n

[drdv]

help: VFLAGS := -v SUB='<L:0>target-1:renamed:'
help: ## Show this help
	@awk $(VFLAGS) -f makefile-doc.awk $(MAKEFILE_LIST)

target-1: ##

produces:

-----------------------
Available targets:
-----------------------
help      Show this help
renamed  -----------------------

There is not much point in setting L:0 if there are no values but it is a valid setting ... we have to handle it.