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

@@ -1517,7 +1517,7 @@ static int local_ast_moh_start(struct ast_channel *chan, const char *mclass, con
 					ast_set_flag(mohclass, MOH_RANDOMIZE);
 				else if (!strcasecmp(tmp->name, "sort") && !strcasecmp(tmp->value, "alpha"))
 					ast_set_flag(mohclass, MOH_SORTALPHA);
-				else if (!strcasecmp(tmp->name, "format")) {
+				else if (!strcasecmp(tmp->name, "format") && !ast_strlen_zero(tmp->value)) {
 					ao2_cleanup(mohclass->format);
 					mohclass->format = ast_format_cache_get(tmp->value);
 					if (!mohclass->format) {
@@ -1831,7 +1831,7 @@ static int load_moh_classes(int reload)
 				ast_set_flag(class, MOH_RANDOMIZE);
 			} else if (!strcasecmp(var->name, "sort") && !strcasecmp(var->value, "alpha")) {
 				ast_set_flag(class, MOH_SORTALPHA);
-			} else if (!strcasecmp(var->name, "format")) {
+			} else if (!strcasecmp(var->name, "format") && !ast_strlen_zero(var->value)) {
 				ao2_cleanup(class->format);
 				class->format = ast_format_cache_get(var->value);
 				if (!class->format) {