res_musiconhold: Avoid spurious warning when 'format' is the empty string

The change to res_config_odbc that allowed empty strings to be
returned to realtime consumers¹ causes a warning to be emitted when
loading MoH classes. So we need to treat an empty 'format' as if it
was not specified to avoid the warning.

ASTERISK-28735 #close
Reported by: Ross Beer

[1] https://gerrit.asterisk.org/c/asterisk/+/13722

Change-Id: I9a271d721e1a0973e80ebe7d75b46a0d8fa0e5a5

res/res_musiconhold.c

@@ -1119,7 +1119,7 @@ static void moh_parse_options(struct ast_variable *var, struct mohclass *mohclas
 			} else if (!strcasecmp(var->value, "randstart")) {
 				ast_set_flag(mohclass, MOH_RANDSTART);
 			}
-		} else if (!strcasecmp(var->name, "format")) {
+		} else if (!strcasecmp(var->name, "format") && !ast_strlen_zero(var->value)) {
 			ao2_cleanup(mohclass->format);
 			mohclass->format = ast_format_cache_get(var->value);
 			if (!mohclass->format) {