Skip initializing preview if not configured

[bene-starzengruber]

Is your feature request related to a problem? Please describe.
Currently, the image resource is loaded again for the preview even if no preview is configured.

Describe the solution you'd like
If the preview configuration is falsy, it should early-exit and not load the image.

Describe alternatives you've considered
Not sure if there are any workarounds - it seems like the code-path is unavoidable at the moment.

Additional context
The images we work with are rather big and we do not always need a preview. For these cases it would be nice to save the call.
I can provide a PR if the change is welcomed 😄

[fengyuanchen]

If you omit the preview option, then they are not any previewing images.

[fengyuanchen]

If you want to know why there are multiple image requests, see #763.