Fix validation when pasting images. (#2436)

Before introducing url / src validation [we allowed
nulls](https://github.com/tldraw/tldraw/pull/2428/files#diff-28a9f0a3d4797d7addd1d61d336fc00acdec878273ac09f048e2e2dfaf77a037L30)
for src so I updated validations to reflect that.

Fixes [#2437](https://github.com/tldraw/tldraw/issues/2437)

### Change Type

- [x] `patch` — Bug fix
- [ ] `minor` — New feature
- [ ] `major` — Breaking change
- [ ] `dependencies` — Changes to package dependencies[^1]
- [ ] `documentation` — Changes to the documentation only[^2]
- [ ] `tests` — Changes to any test code only[^2]
- [ ] `internal` — Any other changes that don't affect the published
package[^2]
- [ ] I don't know

[^1]: publishes a `patch` release, for devDependencies use `internal`
[^2]: will not publish a new version

### Test Plan

1. Copy an image in tldraw in one browser (`cmd + c`)
2. Paste it to a different browser (or at least private mode of the same
browser)
3. Paste should work and you should not see any errors in the console.

- [ ] Unit Tests
- [ ] End to end tests

### Release Notes

- Fixes url validations.

---------

Co-authored-by: alex <alex@dytry.ch>

packages/tlschema/src/assets/TLImageAsset.ts

@@ -27,7 +27,7 @@ export const imageAssetValidator: T.Validator<TLImageAsset> = createAssetValidat
 		name: T.string,
 		isAnimated: T.boolean,
 		mimeType: T.string.nullable(),
-		src: T.srcUrl,
+		src: T.srcUrl.nullable(),
 	})
 )
 

packages/tlschema/src/assets/TLVideoAsset.ts

@@ -27,7 +27,7 @@ export const videoAssetValidator: T.Validator<TLVideoAsset> = createAssetValidat
 		name: T.string,
 		isAnimated: T.boolean,
 		mimeType: T.string.nullable(),
-		src: T.srcUrl,
+		src: T.srcUrl.nullable(),
 	})
 )