CONF_modules_load_file_ex(): Do not try to load an empty file name

Fixes #21258

Reviewed-by: Paul Dale <pauli@openssl.org>
Reviewed-by: Dmitry Belyavskiy <beldmit@gmail.com>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from #21282)

(cherry picked from commit 8b7d5ea)

crypto/conf/conf_mod.c

@@ -184,15 +184,21 @@ int CONF_modules_load_file_ex(OSSL_LIB_CTX *libctx, const char *filename,
     CONF *conf = NULL;
     int ret = 0, diagnostics = 0;
 
+    ERR_set_mark();
+
     if (filename == NULL) {
         file = CONF_get1_default_config_file();
         if (file == NULL)
             goto err;
+        if (*file == '\0') {
+            /* Do not try to load an empty file name but do not error out */
+            ret = 1;
+            goto err;
+        }
     } else {
         file = (char *)filename;
     }
 
-    ERR_set_mark();
     conf = NCONF_new_ex(libctx, NULL);
     if (conf == NULL)
         goto err;