hostapd: fix radius problem due to invalid attributes

The offending commit caused the configuration file to contain:
  radius_auth_req_attr=
  radius_acct_req_attr=
which cause hostapd to add an ATTR of type 0 into the messages.

hostapd: RADIUS message: code=4 (Accounting-Request) identifier=0 length=93
hostapd:    Attribute 40 (Acct-Status-Type) length=6
hostapd:       Value: 7
hostapd:    Attribute 30 (Called-Station-Id) length=28
hostapd:       Value: 'C4-41-1E-F5-2D-55:OpenWifi'
hostapd:    Attribute 61 (NAS-Port-Type) length=6
hostapd:       Value: 19
hostapd:    Attribute 0 (?Unknown?) length=3    <----------------
hostapd:    Attribute 55 (Event-Timestamp) length=6
hostapd:       Value: 1622726457
hostapd:    Attribute 41 (Acct-Delay-Time) length=6
hostapd:       Value: 0
hostapd:    Attribute 44 (Acct-Session-Id) length=18
hostapd:       Value: '9B5961E7235AAEC6'

Fixes: 3bd6c8c728e (hostapd: add additional radius options)
Signed-off-by: John Crispin <[email protected]>

package/network/services/hostapd/files/hostapd.sh

@@ -490,11 +490,11 @@ append_hs20_conn_capab() {
 }
 
 append_radius_acct_req_attr() {
-	append bss_conf "radius_acct_req_attr=$1" "$N"
+	[ -n "$1" ] && append bss_conf "radius_acct_req_attr=$1" "$N"
 }
 
 append_radius_auth_req_attr() {
-	append bss_conf "radius_auth_req_attr=$1" "$N"
+	[ -n "$1" ] && append bss_conf "radius_auth_req_attr=$1" "$N"
 }
 
 append_airtime_sta_weight() {