When subscribed to a single symbol, why is OnData() being called for a date when no data is available?

For example, for AAPL on 2020-08-31, OnData() is being called but there was no trade on 2020-08-30 since the markets were closed.

Also, how would we distinguish this case from the case where a data bar is really missing and therefore we might have to eliminate that symbol from our universe for a while since the indicators will not be accurate?

 

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