join/1: respect empty strings (fix jqlang#668)

builtin.c

@@ -1065,7 +1065,7 @@ static const char* const jq_builtins[] = {
   "def scalars: select(. == null or . == true or . == false or type == \"number\" or type == \"string\");",
   "def scalars_or_empty: select(. == null or . == true or . == false or type == \"number\" or type == \"string\" or ((type==\"array\" or type==\"object\") and length==0));",
   "def leaf_paths: paths(scalars);",
-  "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));",
   "def flatten: reduce .[] as $i ([]; if $i | type == \"array\" then . + ($i | flatten) else . + [$i] end);",
   "def flatten($x): reduce .[] as $i ([]; if $i | type == \"array\" and $x > 0 then . + ($i | flatten($x-1)) else . + [$i] end);",
   "def range($x): range(0;$x);",

tests/all.test

@@ -327,6 +327,10 @@ join(",","/")
 "a,b,c,d"
 "a/b/c/d"
 
+[.[]|join("a")]
+[[""],["",""],["","",""]]
+["","a","aa"]
+
 # Same check for flatten/1
 flatten(-1,3,2,1)
 [0, [1], [[2]], [[[3]]]]