Handle outline overlays better when cycling in outline.el

Handle outline overlays better when cycling in outline.el

* lisp/outline.el (outline--cycle-state): Only consider outline
overlays that are on outline headings; when subtree end is
point-max, return overlay-end +1 because final subtree overlay
only reaches point-max -1 (bug#41198).
(outline-cycle-buffer): Check that buffer has top-level headings
before calling outline-hide-sublevels 1 thus preventing
disconcerting buffer state of content reduced to single "..."