Makefiles: Fix "make depend" with add_custom_command DEPFILE

Since commit cfd8a5ac1f (Makefiles: Add support of DEPFILE for
add_custom_command, 2020-12-04, v3.20.0-rc1~237^2~1) we store in
`CMAKE_DEPENDS_DEPENDENCY_FILES` an empty string as the source file with
which the dependencies of a custom command depfile are associated. When
this list is later expanded by `make depend`, the empty element is
removed and breaks list indexing.  Fix the list expansion to preserve
empty elements.