__str__ function for class mp_position broken

The __str__ function of the class `mp_position` is broken if the members of the class are (still) of type None. Happens for example if the NMEAGPS module has not yet received GGA messageas and the members are all of value None.

Locate the `mp_position` class and inspect its `__str__` function, then reproduce the case where the NMEAGPS module has not received a GGA message and its members remain `None`. Done means string conversion no longer breaks when those values are `None`.

Written by the indexing model from the issue text.