HPCC-22079 Avoid crash in getDaliLayoutInfo for blank ECL definition

Signed-off-by: Gavin Halliday <gavin.halliday@lexisnexis.com>

common/thorhelper/thorcommon.cpp

@@ -2025,7 +2025,7 @@ extern THORHELPER_API IOutputMetaData *getDaliLayoutInfo(IPropertyTree const &pr
             props.getProp("ECL", layoutECL);
             MultiErrorReceiver errs;
             Owned<IHqlExpression> expr = parseQuery(layoutECL.str(), &errs);
-            if (errs.errCount() == 0)
+            if (expr && (errs.errCount() == 0))
             {
                 if (props.hasProp("_record_layout"))  // Some old indexes need the payload count patched in from here
                 {