wifi-scripts: ucode: fix wpa_supplicant mesh

wifi-scripts: ucode: fix wpa_supplicant mesh

Mesh interfaces may not define an encryption setting. This results in
a ucode runtime error when parse_encryption() is called:
  Reference error: left-hand side expression is null
  In parse_encryption(), file /usr/share/ucode/wifi/iface.uc, line 20, byte 32:
    called from function setup_sta (/usr/share/ucode/wifi/supplicant.uc:59:31)
    called from function generate (/usr/share/ucode/wifi/supplicant.uc:243:41)
    called from function setup (./mac80211.sh:254:61)
    called from anonymous function (./mac80211.sh:342:14)
   `    config.auth_type = encryption[0] ?? 'none';`
    Near here ------------------------^

Link: https://github.com/openwrt/openwrt/pull/21293
Signed-off-by: Nick Hainke <vincent@systemli.org>