hist_char2type: Return '/' for NUL.

During -S execution, ccline.cmdfirstc is NULL, so calling histadd() with
a hist-name of '' would fail.  Mimic Vim's behavior and fallback to '/'
in this scenario, since '' is a documented hist-name as of
vim/vim@3e496b0e.

src/nvim/ex_getln.c

@@ -4396,6 +4396,7 @@ static HistoryType hist_char2type(const int c)
     case '>': {
       return HIST_DEBUG;
     }
+    case NUL:
     case '/':
     case '?': {
       return HIST_SEARCH;