Fix small bug in 'completion-table-subvert'.

Fix small bug in 'completion-table-subvert'.

Even for a trivial underlying completion table (where the 'boundaries'
action returns nil), we need to provide nontrivial boundaries so that
they match the behavior of 'all-completions'.

* lisp/minibuffer.el (completion-table-subvert): Return boundaries
even for trivial underlying completion table.
* test/lisp/minibuffer-tests.el (completion-table-subvert-test):
Amend unit test to also test boundaries.  While there, also test
the other completion functions.