Don't update/upload null data

[Philskidvd]

Hello,
I have been trying to build a GPS tracker for my car.
I found a very useful tutorial online, and have built what is a very precise, most of the time.
Data is taken from a GPS board,onto my Pi3, and parsed by NMEA, filtered, and uplaoded to my firebase database.
My problem is that occasionally, the gps returns zero data (ie, at cold boot, or when passing through a tunnel), it's parsed, and uploaded, causing my location to jump to Lat 0 Lng 0.
It appears to be a very simple piece of code, but my attempts at modifying it, to not update the database when this happens, have not worked.

firebase=pyrebase.initialize_app(firebaseConfig)
db=firebase.database()

while True:
        port="/dev/ttyAMA0"
        ser=serial.Serial(port, baudrate=9600, timeout=0.5)
        dataout = pynmea2.NMEAStreamReader()
        newdata=ser.readline()
        n_data = newdata.decode('latin-1')
        if n_data[0:6] == '$GPRMC':
                newmsg=pynmea2.parse(n_data)
                lat=newmsg.latitude
                lng=newmsg.longitude
                gps = "Latitude=" + str(lat) + " and Longitude=" + str(lng)
                print(gps)
                data = {"LAT": lat, "LNG": lng}
                db.update(data)
                print("Data sent")

I am at a very basic level, and would be very grateful for any help you could provide.
Thanks in advance
Phil