Symlink installs do not copy empty directories

[EzraBrooks]

I've noticed that when using --symlink-install, empty directories don't end up getting created in the resulting installation folder. This is sort of an odd case, but in our application we have a system where users can add some files to a particular directory in a ROS package via our UI. If they create that ROS package from scratch and they don't put anything in that directory to start with (which is fairly reasonable), the symlink install (which is our recommended default for users) does not create that directory at build time.

[EzraBrooks]

Seems somewhat likely that this is because creating the directory is not done until here:

ament_cmake/ament_cmake_core/cmake/symlink_install/ament_cmake_symlink_install.cmake.in

Lines 90 to 94 in 08de943

	# ensure that destination exists
	get_filename_component(symlink_dir "${symlink}" PATH)
	if(NOT EXISTS "${symlink_dir}")
	file(MAKE_DIRECTORY "${symlink_dir}")
	endif()

Which is within a for-each loop that is operating on files, thus would not trigger if the directory is empty.