Make nexpi PV optional so that it can be used on eiger's without extg

[RJCD-Diamond]

Fixes: #1117

Required to move ADOdin Eiger into dls-dodal

[coretl]

Shouldn't this happen in DiamondLightSource/dodal#1745 instead?

[RJCD-Diamond]

@coretl The nexpi pv is in the fastcs part of the eiger, unless you want me to move the fastcs odin to dodal as well?

The other option is making it optional, as I see you have agreed in the #1117 but I'm not aware of how to do this neatly

[coretl]

@coretl The nexpi pv is in the fastcs part of the eiger, unless you want me to move the fastcs odin to dodal as well?

Ah, I get it now. I agree the change needs to be made here.

The other option is making it optional, as I see you have agreed in the #1117 but I'm not aware of how to do this neatly

You should just be able to replace the line with:

    nexpi: Optional[SignalRW[int]]

and the signal will not be created if it doesn't exist. If this doesn't work then we will fix that bit.

[RJCD-Diamond]

nexpi: Optional[SignalRW[int]] was failing lint - so changed it to nexpi: SignalRW[int] | None