af_join_many() would throw an exception when passed empty arrays. This was inconsistent with af_join().
With this PR, af_join_many() will ignore empty arrays.

• Is this a new feature or a bug fix?
  bug fix
• More detail if necessary to describe all commits in pull request.
  Now that empty arrays are allowed, the first non-empty array must be used for assertions.
  Internal API has slightly changed to accept pre-allocated array. More functions will be moving to this style.
• Future changes not implemented in this PR.
  internal API of af_join() doesn't yet match af_join_many(&out)

Fixes: #3209

Changes to Users

No changes to existing code should be necessary, but join() with multiple arrays will now handle empty arrays.

Checklist

• Rebased on latest master
• Code compiles
• Tests pass
• Functions added to unified API
• Functions documented