Disable comment_line for ocaml/coq

OCaml does not have line comments, and as far as I can tell neither
does Coq. Setting it to '' (like markdown and html do) throws an error
that can be handled or displayed instead of inserting the default '#'.

rc/tools/comment.kak

@@ -1,4 +1,5 @@
 # Line comments
+# If the language has no line comments, set to ''
 declare-option -docstring "characters inserted at the beginning of a commented line" \
     str comment_line '#'
 
@@ -97,6 +98,7 @@ hook global BufSetOption filetype=markdown %{
 }
 
 hook global BufSetOption filetype=(ocaml|coq) %{
+    set-option buffer comment_line ''
     set-option buffer comment_block_begin '(* '
     set-option buffer comment_block_end ' *)'
 }