Fix controlled component warning

Mm right, when we first render the output, `imageUrl` is `undefined`, so the output textbox renders with `value={undefined}`, which is an "uncontrolled" component that the DOM is free to change.

In practice, this isn't an issue because the textbox has `isReadOnly`, so the user can't _actually_ change it. But it's still a good idea for consistency and clarity to use an empty string instead of `undefined`, and it removes warning spam from my console!

src/app/OutfitUrlsPage.js

@@ -156,7 +156,7 @@ function SingleImageConverter() {
     }
   );
 
-  const imageUrl = data?.outfit?.imageUrl;
+  const imageUrl = data?.outfit?.imageUrl || "";
 
   const previewBackground = useColorModeValue("gray.200", "whiteAlpha.300");
   const spinnerSize = useBreakpointValue({ base: "md", md: "sm" });