Handle the Sentinel case in node creation

We really shouldn't get a sentinel node here but if we for some reason
do we need/should handle it by erroring out rather than propagating the
sentinel.

Reviewed by vraghavan@

src/backend/gpopt/translate/CTranslatorDXLToPlStmt.cpp

@@ -4432,6 +4432,13 @@ CTranslatorDXLToPlStmt::PplanDML
 			aclmode = ACL_INSERT;
 			break;
 		}
+		case gpdxl::EdxldmlSentinel:
+		default:
+		{
+			GPOS_RAISE(gpdxl::ExmaDXL, gpdxl::ExmiDXL2PlStmtConversion,
+				GPOS_WSZ_LIT("Unexpected error during plan generation."));
+			break;
+		}
 	}
 	
 	IMDId *pmdidTargetTable = pdxlop->Pdxltabdesc()->Pmdid();