[fix] fix NPE in SysVariableDesc::equal (#7766)

fe/fe-core/src/main/java/org/apache/doris/analysis/SysVariableDesc.java

@@ -32,6 +32,8 @@ import org.apache.doris.thrift.TStringLiteral;
 
 import com.google.common.base.Strings;
 
+import java.util.Objects;
+
 // System variable
 // Converted to StringLiteral in analyze, if this variable is not exist, throw AnalysisException.
 public class SysVariableDesc extends Expr {
@@ -189,6 +191,7 @@ public class SysVariableDesc extends Expr {
         if (!setType.equals(((SysVariableDesc) obj).getSetType())) {
             return false;
         }
-        return literalExpr.equals(((SysVariableDesc) obj).getLiteralExpr());
+
+        return Objects.equals(literalExpr, ((SysVariableDesc) obj).getLiteralExpr());
     }
 }