[pdata] Optional field accessors don't provide a way to clear the optional value

[dmitryax]

Optional fields (e.g. HistogramDataPoint.Max) can be empty, but there is no way to set the field to the empty state with the interface that pdata provides.

[dmitryax]

@bogdandrutu @codeboten let me know if you agree that it's needed

[bogdandrutu]

This is an addition, so I would not add it until a clear need for it.