product more robust against empty input

lib/pure/algorithm.nim

@@ -148,6 +148,7 @@ proc product*[T](x: openarray[seq[T]]): seq[seq[T]] =
   var next: seq[T] = @[]
   next.setLen(x.len)
   for i in 0..(x.len-1):
+    if len(x[i]) == 0: return
     initial[i] = len(x[i])-1
   indexes = initial
   while true:

tests/stdlib/talgorithm.nim

@@ -1,6 +1,11 @@
 import unittest
+import algorithm
 
 suite "product":
+  test "empty input":
+    check product[int](newSeq[seq[int]]()) == newSeq[seq[int]]()
+  test "bit more empty input":
+    check product[int](@[newSeq[int](), @[], @[]]) == newSeq[seq[int]]()
   test "a simple case of one element":
     check product(@[@[1,2]]) == @[@[1,2]]
   test "two elements":