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

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]]]]