return something sensible with insensible input of no constraints

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/vegan/pkg/vegan@2634 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
permutest.ccalist also accepts a null model with no constraints for comparison

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/vegan/pkg/vegan@2635 edb9625f-4e0d-4859-8d74-9fd3b1da38cb