FS-8537: Passing nil to various lua functions causes segfault

Various functions exposed via lua do not check their parameters for null
causing freeswitch to segfault.

This change adds checking for null parameters and returns an error
instead of segfaulting.

src/switch_utils.c

@@ -996,6 +996,11 @@ SWITCH_DECLARE(switch_bool_t) switch_simple_email(const char *to,
 	switch_bool_t rval = SWITCH_FALSE;
 	const char *err = NULL;
 
+	if (zstr(to)) {
+		err = "No to address specified";
+		goto end;
+	}
+
 	if (!zstr(file) && !zstr(convert_cmd) && !zstr(convert_ext)) {
 		if ((ext = strrchr(file, '.'))) {
 			dupfile = strdup(file);