cron: permit entry.schedule() to reuse existing

app/modules/cron.c

@@ -135,10 +135,13 @@ static size_t lcron_findindex(lua_State *L, cronent_ud_t *ud) {
 
 static int lcron_schedule(lua_State *L) {
   cronent_ud_t *ud = luaL_checkudata(L, 1, "cron.entry");
-  char *strdesc = (char*)luaL_checkstring(L, 2);
+  char *strdesc = (char*)luaL_optstring(L, 2, NULL);
+
+  if (strdesc != NULL) {
     struct cronent_desc desc;
     lcron_parsedesc(L, strdesc, &desc);
     ud->desc = desc;
+  }
 
   size_t i = lcron_findindex(L, ud);
 

docs/modules/cron.md

@@ -83,10 +83,10 @@ end)
 
 ## cron.entry:schedule()
 
-Sets a new schedule mask.
+Sets a new schedule mask and/or restores an unscheduled entry.
 
 #### Syntax
-`schedule(mask)`
+`schedule([mask])`
 
 #### Parameters
 - `mask` - [crontab](https://en.wikipedia.org/wiki/Cron#Overview)-like string mask for schedule