add nullptr check to createTable() method as done for CSCs, MDTs and TGCs

Hi,

this MR adds a nullptr check to the createTable() method of the MM_RegionSelectorTable.cxx and sTGC_RegionSelectorTable.cxx as it is also done for the CSCs, MDTs and TGCs already. The nullptr check can become important in case an identifier dictionnary in the IdDictParser package contains identifiers to detector elements which are not part of a layout. Without the nullptr check, the code would cause a segmentation violation.

Best, Nico

Adding @morgens and @markowen for the trigger Adding @rosati, @wleight and @cchau for MuonSW