handle empty comments correctly

since the lines in the parser have all newline characters removed, \s+ will
not match empty comments.

Added an example that makes other tests fail without this patch.

test/fixtures/comment.po

@@ -19,3 +19,11 @@ msgstr ""
 #. Extracted comment
 msgid "Title, as plain text"
 msgstr "Attribut title, en tant que texte brut"
+
+#
+#.
+#:
+#, fuzzy
+msgid "Empty comment"
+msgstr "Empty"
+