[FIX] searches with filters using "empty" domain/context values (but not attributes)

[FIX] searches with filters using "empty" domain/context values (but not attributes)

e.g. @domain="[]" would be seen as non-empty by the search view, and
if multiple domains the search view would generate a nonliteral
``['|', '[]', '[]]`` which would just yield ``['|']`` after evaluation
and concatenation, which is an invalid domain and would blow up the
server.

Specifically filter out the values ``[]`` and ``{}`` from filters

bzr revid: xmo@openerp.com-20121129112413-yrgncnesqs093jwf