Avoid crashing when a tiled scene texture is provided by an extension

[uclaros]

Description

Handle the case when texture.source is undefined, because the texture is provided by an extension, eg. EXT_texture_webp

According to spec:

When texture.source is undefined, the image SHOULD be provided by an extension or application-specific means, otherwise the texture object is undefined.

fixes: #57024

[github-actions]

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