[improvement](statistics)Use defaultSessionVariable instead of clone a new one. (#34672) (#35531)

backport https://github.com/apache/doris/pull/34672

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fe/fe-core/src/main/java/org/apache/doris/statistics/util/StatisticsUtil.java

@@ -755,7 +755,7 @@ public class StatisticsUtil {
     }
 
     protected static SessionVariable findConfigFromGlobalSessionVar(String varName) throws Exception {
-        SessionVariable sessionVariable =  VariableMgr.newSessionVariable();
+        SessionVariable sessionVariable =  VariableMgr.getDefaultSessionVariable();
         VariableExpr variableExpr = new VariableExpr(varName, SetType.GLOBAL);
         VariableMgr.getValue(sessionVariable, variableExpr);
         return sessionVariable;