Set glonass frequency to None if not Glonass measurement

[gijskoning]

Currently, the frequency is set to np.nan in raw_gnss.py but None would be better since the type can then be Optional[Int] in GnssMeasurements.
The function read_raw_qcom is used in a private repo. Need to verify it correctly works there too.

laika/laika/raw_gnss.py

Line 203 in bcd8985

glonass_freq = (i.glonassFrequencyIndex - 7) if report.source == 1 else np.nan