Lockscreen Targets : Fix empty magnetic behaviour

When no valid targets are set, set settings to null in order
to enable magnetic targets (aosp behavior)

Change-Id: I0ccaad25cc85f55b99e4c34cb4ffe2d06bda4a80
Merge "Lockscreen Targets : Fix empty magnetic behaviour" into cm-10.2
Merge remote-tracking branch 'upstream/cm-10.2' into HEAD