AccountBalance throws exception on missing data

[jimbkim]

in the account.py file, I updated the data class as follows because the API doesn't return these keys anymore, although it's still int he docs.

apex_starting_day_margin_equity: Decimal = Field(default = None)
buying_power_adjustment: Decimal = Field(default = None)
buying_power_adjustment_effect: PriceEffect = Field(default = PriceEffect.NONE)

I tried using Option, but didn't seem to work.

[Graeme22]

The keys are still present for me, but looks like they're not guaranteed to be present! I'll make them optional. Thanks for identifying this!