Accepts missing value in filter expr, mkjsf-16

Accepts missing value in filter expr, mkjsf-16

Would raise JSF/Ajax error on SearchCommand.filter attribute when
entering 'pz:id=' (for instance). This is a legitimate interim entry
however, since the user could leave the field in that state to cut and
paste a target ID from somewhere else.

A value of 'pz:id=' at time of search would currently result in
'no targets' error from pazpar2.