test :random-search needs to check if expected is nil

[Yehouda]

in spatial-tree-test.lisp, the test :random-search sometimes fails because the rectangle it searches for is really not in the tree, so the value of the variable expected is nil, and result is nil. Currently that is signaled as failure. It should do something like:

(if expected (is-true result) (is-false result))

Can also skip the set-difference tests when expected is nil, but these tests pass anyway.