fix boolean search attr method

Testcase by @mlocati:
- added a new boolean page attribute
- created three pages
  - page1 without this attribute
  - page2 with this attribute, unchecked
  - page3 with this attribute, checked
- in the page search form (website/dashboard/sitemap/search/) I added this new attribute to the custom search
  - leaving the attribute checkbox unchecked: no page is found
  - checking the attribute checkbox: only page3 is found

I had to change the default value to null as 0 isn't the same as "null". To me:
- null should mean attr isn't selected
- 0 means attr is selected but not ticked
- 1 means selected and ticked