Parsing fails on empty ERB comments

[Harry Maclean]

The following code, a comment block with no comment text, will fail to parse:

<%# %>

(template [0, 0] - [1, 0]
  (comment_directive [0, 0] - [1, 0]
    (comment [0, 3] - [1, 0])))
example.erb	0 ms	(MISSING "%>" [1, 0] - [1, 0])