Fix Cartesian product to return empty set if empty set is given (#503)

This fixes #499, where a matrix strategy with only include keys ends up
causing multiple builds.  This bugs appears to have been introduced in #415,
when extra include keys are added in the matrix strategy.  The cause
seems to be because the CartesianProduct function returns an item with
empty keys, instead of return an empty set.

Co-authored-by: Ed Tan <edtan@users.noreply.github.com>

act/common/cartesian.go

@@ -27,7 +27,7 @@ func cartN(a ...[]interface{}) [][]interface{} {
 	for _, a := range a {
 		c *= len(a)
 	}
-	if c == 0 {
+	if c == 0 || len(a) == 0 {
 		return nil
 	}
 	p := make([][]interface{}, c)

act/common/cartesian_test.go

@@ -25,4 +25,16 @@ func TestCartesianProduct(t *testing.T) {
 		assert.Contains(v, "baz")
 	}
 
+  input = map[string][]interface{}{
+		"foo": {1, 2, 3, 4},
+		"bar": {},
+		"baz": {false, true},
+  }
+  output = CartesianProduct(input)
+  assert.Len(output, 0)
+
+  input = map[string][]interface{}{}
+  output = CartesianProduct(input)
+  assert.Len(output, 0)
 }
+