Before loading the data and fields from the data source, write a custom SQL statement to get those values from tables without nulls.
If it is a single table then you can write this code to get the table contents without null values.
SELECT * FROM table WHERE field_name IS NOT 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