Before loading the data and fields from data source, write a custom sql statement to get those values from tables with nulls and then replace them.
If it is a single table then you can write this code to get the table contents updated values instead of null.
UPDATE TABLE MESSAGES SET User_email_id = "old" where User_email_id IS NULL
The new table can then be used to perform visualization.
For multiple tables you should rewrite the code to fetch each table. You can then rename the query after creating and use the data.
The custom sql table later joined with other tables just like actual tables.
hope this helps you