func_cdr: Allow empty value for CDR dialplan function.

A regression was introduced in 12 where passing an empty value
to the CDR dialplan function was not longer allowed. This
change returns to the behavior of 11 where it is permitted.

ASTERISK-26173

Change-Id: I3f148203b54ec088007e29e30005a5de122e51c5

funcs/func_cdr.c

@@ -377,7 +377,7 @@ static void cdr_write_callback(void *data, struct stasis_subscription *sub, stru
 			payload->cmd, payload->cmd);
 		return;
 	}
-	if (ast_strlen_zero(payload->value)) {
+	if (!payload->value) {
 		ast_log(AST_LOG_WARNING, "%s requires a value (%s(variable)=value)\n)",
 			payload->cmd, payload->cmd);
 		return;