Always set version variable of find_dependency macro

If there is no ARGV1, that is fine; version will be made empty, and no
version will be passed to find_package().

This is relevant when find_dependency is invoked multiple times,
sometimes with a version specified and sometimes without.

 find_dependency(dep1 3.4)
 find_dependency(dep2) # version still set to 3.4.
Merge topic 'fix-find_dependency-macro' into next

5878c5d9 Always set version variable of find_dependency macro