Allow some heads with no hints

* hydra.el (hydra--hint): If the HINT part of HEAD is explicitely nil,
  omit it from the compound hint.

Example:

    (global-set-key
     (kbd "C-M-o")
     (defhydra hydra-window (:color amaranth)
       "window"
       ("h" windmove-left nil)
       ("j" windmove-down nil)
       ("k" windmove-up nil)
       ("l" windmove-right nil)
       ("v" (lambda ()
              (interactive)
              (split-window-right)
              (windmove-right))
            "vert")
       ("x" (lambda ()
              (interactive)
              (split-window-below)
              (windmove-down))
            "horz")
       ("q" nil "cancel")))

Here, "h", "j", "k", "l" will not be in the echo area.

hydra.el

@@ -225,7 +225,12 @@ It's intended for the echo area, when a Hydra is active."
               (propertize
                (car h) 'face
                (hydra--face h body-color))))
-           heads ", ")))
+           (cl-remove-if
+            (lambda (x)
+              (and (> (length x) 2)
+                   (null (cl-caddr x))))
+            heads)
+           ", ")))
 
 (defun hydra-disable ()
   "Disable the current Hydra."