mt_wifi lua detect wanif name not null (#5748)

package/lean/mt/drivers/mt_wifi/files/mt7615.lua

@@ -26,7 +26,7 @@ end
 function add_vif_into_lan(vif)
     local mtkwifi = require("mtkwifi")
     local wanif = mtkwifi.__trim(mtkwifi.read_pipe("uci get network.wan.ifname"))
-    if (string.match(vif, wanif)) then
+    if ('' ~= wanif and string.match(vif, wanif)) then
         return
     end
     local brvifs = mtkwifi.__trim(