join/1 should respect empty strings in input

[nicowilliams]

The join/1 builtin needs help:

-def join($x): reduce .[] as $i (""; . + (if . == "" then $i else $x + $i end));
+def join($x): reduce .[] as $i (null; (.//"") + (if . == null then $i else $x + $i end));

See also comments on #552.

[joelpurra]

@nicowilliams: join fails for an empty input array; see pull request #669.

[joelpurra]

This issue was fixed by #669 cca3e92. Thanks!