Prediction edge case when deleting self.history to reduce model size

From #1159, a solution for reducing the model size was to remove information that isn't needed at prediction time like `self.history`. This has been working well for me except when there is only one date to predict. The issue is `_sample_uncertainty` can't calculate the average date diff when only a single date is provided, so it tries to infer the diff using `self.history`.

https://github.com/facebook/prophet/blob/3c5321681a82a52d1c9b34725fc82481b7adec50/python/prophet/forecaster.py#L1617-L1618

A workaround for now is keeping two elements in `self.history` instead of clearing everything. However, it would be nice if there was a solution that would allow all history to be cleared.