Issue 12: Make possible to generate filters to null values
Status:  New
Owner: ----
Reported by betoneto.tbo@gmail.com, May 14, 2013
What steps will reproduce the problem?
1. Configure a FilterTable with some filter for a Date type
2. At the FilterGenerator create a IsNull filter when the value is null
3. Clear the filter value after enter some value (date range)
4. Is not possible to intercept the null value and generate a IsNull filter

What is the expected output? What do you see instead?
- the FilterTable ignores null filter values and doesn't fire the "generateFilter" of FilterGenerator

What version of the product are you using? On what operating system?
- FilterTable 0.8.8.v7 
- Vaadin 7.0.5
- Windows 7 PRO x64
- Tomcat 7

At my scene, I need to create a IsNull filter for a Date type property when the user cleared the range.

Workarround: recreate the DateFilterPopup component and intercept the null range to create a IsNull filter at the generateFilter method.