private Expression parseIsNullExpression(ExpressionContext context, boolean canUseFullPath) {
    return new IsNullExpression(
        parseExpression(context.unaryBeforeIsNullExpression, canUseFullPath),
        context.operator_not() != null);
  }
function for parsing file path used by LOAD statement.
iotdb-core/datanode/src/main/java/org/apache/iotdb/db/queryengine/plan/parser/ASTVisitor.java