CREATING A SECOND DATAFRAME WITH EVENTS OCCURED IN FIRST DATAFRAME - spark scala

I have a dataframe df1 with columns "date" in string format and "contracts" in string format and i have to check for two things:

1. If the column date has a date older than one month since the current date.
2. If the column contracts has a NULL value.

If the first condition is true I want to add this event on another dataframe adding the value "NULL" to the column events of the second dataframe. If the second condition is true I want to add also the event "OLD_DATE". The output dataframe should be like this

Output example