A cleanup for MCOL-4464 Bitwise operations not like in MariaDB

CI with RelWithDebInfo builds revealed a problem in the main
patch for MCOL-4464, which did not show up with Debug builds.

Methods like:
- getDoubleVal()
- getDateIntVal()
- getDatetimeIntVal()
- getTimestampIntVal()
- getTimeIntVal()
- getUintVal()
- getIntVal()
- getStrVal()
require the caller to initialize the isNull argument to false.
This fact was not taken into account in MCOL-4464.

Adding proper initializations.

dbcon/execplan/treenode.h

@@ -267,13 +267,13 @@ public:
     }
     datatypes::TUInt64Null toTUInt64Null(rowgroup::Row& row)
     {
-        bool isNull;
+        bool isNull = false;
         uint64_t val = getUintVal(row, isNull);
         return datatypes::TUInt64Null(val, isNull);
     }
     datatypes::TSInt64Null toTSInt64Null(rowgroup::Row& row)
     {
-        bool isNull;
+        bool isNull = false;
         int64_t val = getIntVal(row, isNull);
         return datatypes::TSInt64Null(val, isNull);
     }