Update Image.d.ts (#39230)

Summary:
I've seen that this throws a warning:
```
<ImageBackground source={{ uri: undefined }}>
```

But this is properly working, without a warning, thus, I believe the code is allowing it:
```
<ImageBackground source={isLoading ? undefined : { uri: 'https://....' }}>
```

## Changelog:

[GENERAL] [CHANGED] - Fixed `source` in `Image` type

Pull Request resolved: https://github.com/facebook/react-native/pull/39230

Test Plan: Not necessary. It's just a DOC change

Reviewed By: rshest

Differential Revision: D48939824

Pulled By: NickGerleman

fbshipit-source-id: fca2e07b7b4d4b947e365bb9d82fcf4cea484951

packages/react-native/Libraries/Image/Image.d.ts

@@ -223,7 +223,7 @@ export interface ImagePropsBase
    *
    * The currently supported formats are png, jpg, jpeg, bmp, gif, webp (Android only), psd (iOS only).
    */
-  source: ImageSourcePropType;
+  source?: ImageSourcePropType | undefined;
 
   /**
    * A string representing the resource identifier for the image. Similar to