sysmobts: Do not write "trx-calibration-path (null)" in the config file
sysmobts: Do not write "trx-calibration-path (null)" in the config file

When not specifying a config path, then saving the running config
it would end up as "(null)" and then leads to an error like this:

 <0006> calib_file.c:147 Failed to open '(null)/calib_rxu_850.cfg' for calibration data.

Add a NULL check to avoid this issue when writing the config file.

osmocom/osmo-bts,shimaore/osmo-bts,telenoobie/osmo-bts,shimaore/osmo-bts,geosphere/osmo-bts,osmocom/osmo-bts,shimaore/osmo-bts,telenoobie/osmo-bts,osmocom/osmo-bts,geosphere/osmo-bts,telenoobie/osmo-bts,geosphere/osmo-bts,geosphere/osmo-bts,telenoobie/osmo-bts,shimaore/osmo-bts