Fix bug that the ArithmeticExpr's write method is not implement, causing FE crash when creating function like:
CREATE ALIAS FUNCTION IF NOT EXISTS mesh_udf_test1(INT,INT) WITH PARAMETER(n,d) AS ROUND(1+floor(n/d));
Add if exists and if not exists for drop and create function
Fix a minor bug that if file does not exist, hdfs() table valued function will throw NPE
