tpl/transform: Revert unmarshal whitespace removal

Fixes #12977

tpl/transform/transform_integration_test.go

@@ -261,3 +261,21 @@ disableKinds = ['page','rss','section','sitemap','taxonomy','term']
 <mi>y</mi>
 	`)
 }
+
+// Issue #12977
+func TestUnmarshalWithIndentedYAML(t *testing.T) {
+	t.Parallel()
+
+	files := `
+-- hugo.toml --
+disableKinds = ['page','rss','section','sitemap','taxonomy','term']
+-- layouts/index.html --
+{{ $yaml := "\n  a:\n    b: 1\n  c:\n    d: 2\n" }}
+{{ $yaml | transform.Unmarshal | encoding.Jsonify }}
+`
+
+	b := hugolib.Test(t, files)
+
+	b.AssertFileExists("public/index.html", true)
+	b.AssertFileContent("public/index.html", `{"a":{"b":1},"c":{"d":2}}`)
+}

tpl/transform/unmarshal.go

@@ -112,9 +112,8 @@ func (ns *Namespace) Unmarshal(args ...any) (any, error) {
 	if err != nil {
 		return nil, fmt.Errorf("type %T not supported", data)
 	}
-	dataStr = strings.TrimSpace(dataStr)
 
-	if dataStr == "" {
+	if strings.TrimSpace(dataStr) == "" {
 		return nil, nil
 	}