LegacyStorage: Fix logic to be more robust

config_new(filename) will return nullptr if the filename
points to a non-existing file.  Additionally, if the config
doesn't have an Adapter section it is considered useless
by legacy stack, so copy that logic here too.

Bug: 144376599
Test: manual
Change-Id: I6ceb49f4fa49fb220a40cd3c502f63775859299c