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The doc states that og_context returns "Array keyed by the group type and group ID, or FALSE if no context was found."
My investigation turns up that:
In OG 7.x-2.0-alpha1 it returned FALSE
From OG 7.x-2.0-beta3 it returned NULL (Patch from was committed.)
Since OG 7.x-2.2 it returns an empty array (Patch from was commited.)
I think the behaviour should go back to returning FALSE again.
I know og_comment checks (isset($context)) which only works between OG 7.x-2.0-beta3 and OG 7.x-2.1
Other modules may use different ways to check for an existing context so one is sure to break no matter what we do.