Handle NULL while deriving propagation expression [#143443847]

While constructing the propagation expression in case of queries
involving default list partitions which have indexes, we ended
up with NULLs crashing the database.

Currently, we fallback to planner. But we need to handle it in a
better way in future.
Signed-off-by: NDhanashree Kashid <dkashid@pivotal.io>

libgpopt/src/translate/CTranslatorExprToDXLUtils.cpp

@@ -564,7 +564,7 @@ CTranslatorExprToDXLUtils::UlPartKeyLevel
 //		CTranslatorExprToDXLUtils::PdxlnPartialScanTestRange
 //
 //	@doc:
-// 		Construct a test expression for the given range  
+// 		Construct a test expression for the given range or list
 //		based part constraint
 //
 //---------------------------------------------------------------------------
@@ -589,6 +589,17 @@ CTranslatorExprToDXLUtils::PdxlnPartialScanTestRange
 	else // list partition
 	{
 		IDatum *pdatum = prng->PdatumLeft();
+		if (pdatum == NULL)
+		{
+			// TODO: In case of default partitions, we end up with NULL for Left and Right Datum.
+			// Currently we fallback and should handle it better in future.
+			GPOS_RAISE
+				(
+				gpdxl::ExmaDXL,
+				gpdxl::ExmiExpr2DXLUnsupportedFeature,
+				GPOS_WSZ_LIT("Queries over default list partition that have indexes")
+				);
+		}
 		GPOS_ASSERT(pdatum->FMatch(prng->PdatumRight()));
 
 		CDXLDatum *pdxldatum = Pdxldatum(pmp, pmda, pdatum);