[FRR] fix pfx_filter to fix bgpd.conf.j2 rendering when no vlan interfaces (#2994)

src/sonic-config-engine/sonic-cfggen

@@ -93,6 +93,10 @@ def pfx_filter(value):
        For eg - VLAN_INTERFACE|Vlan1000 vs VLAN_INTERFACE|Vlan1000|192.168.0.1/21
     """
     table = OrderedDict()
+
+    if not value:
+        return table
+
     for key,val in value.items():
         if not isinstance(key, tuple):
             continue