Ensure at least one command if no dependencies

Ensure at least one command if no dependencies

C++Builder's `make.exe` complains if a target has no dependencies (e.g.
after variable expansion) and no lines of commands. Ensure there is a
blank command line if the dependency list is entirely made of variables.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
(Merged from https://github.com/openssl/openssl/pull/13540)