Fix an assertion failure (and a bug) when we write a profile to keyfile, that has a setting that has all values at the default. Keyfile writer would wrongly omit the setting, but the mere presence of a setting can make a difference (and thus writer wrote something different to disk -- in some cases the result can be even invalid, triggering an assertion).
Also, as a second part of this branch, address setting
gsm.apn to empty values. This is for merge request 98.