thermal-core: Fix invalid sensor request not accounted correctly

Add NULL check for sensor name before using it in sensor_get_id and
return NULL explicitly when sensor id is not valid in get_sensor of
thermal core.

Change-Id: I4200a5691183aa2d40ef8ed64554f0beb8ce6d0d
Signed-off-by: Anji Jonnala <anjir@codeaurora.org>