tpl/collections: Fix where ... not in with empty slice

Fixes #13621

tpl/collections/collections_integration_test.go

@@ -278,3 +278,23 @@ disableKinds = ['rss','sitemap', 'taxonomy', 'term', 'page']
 
 	b.AssertFileContentExact("public/index.html", "0: /a3_b1.html\n\n1: /b2.html\n\n2: /a1.html\n\n3: /a2.html\n$")
 }
+
+// Issue 13621.
+func TestWhereNotInEmptySlice(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+-- layouts/home.html --
+{{- $pages := where site.RegularPages "Kind" "not in" (slice) -}}
+Len: {{  $pages | len }}|
+-- layouts/all.html --
+All|{{ .Title }}|
+-- content/p1.md --
+
+`
+
+	b := hugolib.Test(t, files)
+
+	b.AssertFileContent("public/index.html", "Len: 1|")
+}

tpl/collections/where.go

@@ -138,6 +138,9 @@ func (ns *Namespace) checkCondition(v, mv reflect.Value, op string) (bool, error
 		}
 
 		if mv.Len() == 0 {
+			if op == "not in" {
+				return true, nil
+			}
 			return false, nil
 		}
 

tpl/collections/where_test.go

@@ -761,6 +761,7 @@ func TestCheckCondition(t *testing.T) {
 			expect{true, false},
 		},
 		{reflect.ValueOf(123), reflect.ValueOf([]int{45, 678}), "not in", expect{true, false}},
+		{reflect.ValueOf(123), reflect.ValueOf([]int{}), "not in", expect{true, false}},
 		{reflect.ValueOf("foo"), reflect.ValueOf([]string{"bar", "baz"}), "not in", expect{true, false}},
 		{
 			reflect.ValueOf(time.Date(2015, time.May, 26, 19, 18, 56, 12345, time.UTC)),