GN: add sources_when_disabled to optional

tools/fiddle/fiddle_main.cpp

Index: tools/fiddle/fiddle_main.cpp
diff --git a/tools/fiddle/fiddle_main.cpp b/tools/fiddle/fiddle_main.cpp
index bf6a8c27d5fae86f43a0b8bd46626d7a1e694449..0fa61586663073835e4317a9649d227d3d393089 100644
--- a/tools/fiddle/fiddle_main.cpp
+++ b/tools/fiddle/fiddle_main.cpp
@@ -136,10 +136,12 @@ int main() {
     if (options.pdf) {
         SkDynamicMemoryWStream pdfStream;
         sk_sp<SkDocument> document(SkDocument::MakePDF(&pdfStream));
-        srand(0);
-        draw(document->beginPage(options.size.width(), options.size.height()));
-        document->close();
-        pdfData.reset(pdfStream.copyToData());
+        if (document) {
+            srand(0);
+            draw(document->beginPage(options.size.width(), options.size.height()));
+            document->close();
+            pdfData.reset(pdfStream.copyToData());
+        }
     }
     if (options.skp) {
         SkSize size;