Clarify platform behavior when dashboard url is not returned

[tinygrasshopper]

while polling on update.

Have not expressed an opinion on how the broker can clear out an existing dashboard url, but clarified current behaviour in a way that its backwards compatible.

[cfdreddbot]

Hey tinygrasshopper!

Thanks for submitting this pull request! I'm here to inform the recipients of the pull request that you and the commit authors have already signed the CLA.

[pmorie]

One minor nit - there's a misspelling in the commit message ('clarify')

[duglin]

rebase needed

[duglin]

SGTM just waiting for the rebase

[mattmcneeney]

@tinygrasshopper We discussed this on today's call and are happy to start reviewing, but it needs a rebase first!

[tinygrasshopper]

@mattmcneeney rebased

[duglin]

LGTM