Cope with null document

[camelpunch]

This can happen in unit tests, especially when using Jest with jsdom.

You may get warnings like:

    Cannot log after tests are done. Did you forget to wait for something async in your test?
        Attempted to log "Error: Uncaught [TypeError: Cannot read property 'documentElement' of null]

If we check for the existence of the document first, we can tidy up our test output.