libtracker-common: tracker_string_list_to_{gslist|string}() both accept NULL now

Instead of erroring, now we just return NULL when the string list is empty.
libtracker-common, tracker-miner-fs: Make sure we load config files with

When using TRACKER_USE_CONFIG_FILES env var.
Before we would load the GKeyFile but not push that into the GObject config
for each binary.