Simple Question: PHP, MySQL, HTML Form and NULL

I have added a new column in an existing MYSQL table called "event_date" - type: DATE, NULL default. This database field will be filled if and when the field in the HTML form is complete. When I added the new field to MySQL, it automatically assigned NULL to all the existing records - which is what I wanted it to do. I tested the HTML form, and when someone enters a date in the HTML form, the date appears correctly in the table field. Perfect. Now my question: When the HTML form date field is left blank and the form is submitted, instead of putting "NULL" in the MySQL event_date field, I found: 0000-00-00. Is this because the form is submitting an " " to the database field? Is there some kind of if/then statement I should use so that when the field is empty, "NULL" will be entered into that table field? Thank you, Shawna