Add warning if empty modules entry in .app.src

An empty list is required for the modules tuple, as in
{modules, []}
so a sed call could populate the list. This is mentioned in README, but
can be overlooked.

As discussed in https://github.com/ninenines/erlang.mk/pull/85, this isn't immediately fixable. Therefore, if the empty tuple is not present, relx can fail on missing dependency
errors. Therefore, output a warning if the empty tuple isn't found, to
help the user diagnose the problem.