Fix issue on full calendar plugin to check if $calendar_options['events'] isset

Thanks for identifying this. It would be better if there were detailed steps to reproduce, but checking that something isset before attempting to use it is a solid best practice anyway.

Given that the key change is being proposed is to check if the array key isset, I would propose instead to do this as a separate check before attempting to assign it to the $entries variable. It makes for more readable code, as well as making the diff in the proposed change much easier to understand.

Please let me know if this updated patch resolves your issue and I'll be happy to get this merged in.

Merged in the change proposed in #3. Closing this issue but feel free to reopen it if additional work is needed.