Fix Can't convert with callable None - for junos routers

[teresalves]

The code keeps breaking on this line: https://github.com/napalm-automation/napalm/blob/develop/napalm/junos/junos.py#L1339
when there is a key that is not included in the GROUP_FIELDS_DATATYPE_MAP
In particular, for the group names "multihop" and "cluster" this is a problem because it will inevitably show up in the router configs, but the datatype will always be None, which will provoke this error:
ValueError: Can't convert with callable None - no default is defined for this type.
originating from here

In previous versions, this convert function was not doing the type checks, so if it was just returning None. The value for this case is irrelevant because both are deleted from the config further (here and here), respectively. Their existence in the config is temporary and not necessary - it is only used to produce/update other values (here and here)

Therefore, I added here a conditional clause that never adds them to the final bgp config, so there is no need to delete it further ahead. Instead, a separate object is created for validation.

Issue: #1932

[mirceaulinic]

@teresalves sorry for late review, would you mind updating your branch? Thanks.

[teresalves]

Hey @mirceaulinic ! Thanks for taking a look
Updated now, could you recheck please?

[mirceaulinic]

Thanks @teresalves!