Add a new SCT online strip defect in SCT_ReadCalibDataCondAlg

Adding a newly added SCT online strip defect "LO_GAIN_ABSO" in SCT_ReadCalibDataCondAlg. Additionally, in order to avoid failure when a defect undefined in this class shows up, such as the incident reported in https://its.cern.ch/jira/browse/ATLASRECTS-7162, a protection for accessing undefined map key is also added.

Let me tag @csebasti as SCT software coordinator.