tpl/collections: Make dict return nil when no values are provided

tpl/collections/collections.go

@@ -157,7 +157,17 @@ func (ns *Namespace) Delimit(ctx context.Context, l, sep any, last ...any) (stri
 // Dictionary creates a new map from the given parameters by
 // treating values as key-value pairs.  The number of values must be even.
 // The keys can be string slices, which will create the needed nested structure.
+// If no values are provided, nil will be returned.
 func (ns *Namespace) Dictionary(values ...any) (map[string]any, error) {
+	if len(values) == 0 {
+		// A common construct is to do
+		//     {{ $opts := dict }}
+		// And then conditionally assign it if some condition is set.
+		// The only difference between this and an empty map is that this cannot be written to,
+		// which is not something we do in Hugo (or: If we do, that's a bug).
+		// This saves us ~48 bytes in memory allocation on 64-bit architectures.
+		return nil, nil
+	}
 	if len(values)%2 != 0 {
 		return nil, errors.New("invalid dictionary call")
 	}

tpl/collections/collections_integration_test.go

@@ -653,3 +653,21 @@ All.
 		}
 	})
 }
+
+func TestEmptyDictShouldBeNil(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+-- layouts/home.html --
+{{ $d := dict }}
+{{ printf "dict: %T %t %d" $d (eq $d nil) (len $d) }}
+{{ range $d }}FAIL{{ end }}
+index: {{ index $d "foo" }}|
+{{ $d2 := dict "foo" "bar" }}
+{{ $d3 := merge $d $d2 }}
+{{ printf "d3: %v" $d3 }}|
+`
+
+	hugolib.Test(t, files).AssertFileContent("public/index.html", "dict: map[string]interface {} true 0", "! FAIL", "index: |", "d3: map[foo:bar]|")
+}