Fatal typo in line 508 of adafruit_gps.py

[Direct Link to Line 508](https://github.com/adafruit/Adafruit_CircuitPython_GPS/blob/ab6b33a57c0c18972603826a2e3cfe8443b0ed90/adafruit_gps.py#L508C1-L508C25)

Line currently reads
`if data is None:`
Line should read 
`if parsed_data is None:`
to prevent parsed_data for being sent with NoneType. 

causes _read_degrees to try and subscript a NoneType, causes script failure.