[sonic-cfggen]: Fix minigraph DhcpRelays parsing issue (#2154)

* Check that we have dhcprelays text before using it

* Add additional variable to avoid extracting node again and again

src/sonic-config-engine/minigraph.py

@@ -211,8 +211,9 @@ def parse_dpg(dpg, hname):
 
             # If this VLAN requires a DHCP relay agent, it will contain a <DhcpRelays> element
             # containing a list of DHCP server IPs
-            if vintf.find(str(QName(ns, "DhcpRelays"))) is not None:
-                vintfdhcpservers = vintf.find(str(QName(ns, "DhcpRelays"))).text
+            vintf_node = vintf.find(str(QName(ns, "DhcpRelays")))
+            if vintf_node is not None and vintf_node.text is not None:
+                vintfdhcpservers = vintf_node.text
                 vdhcpserver_list = vintfdhcpservers.split(';')
                 vlan_attributes['dhcp_servers'] = vdhcpserver_list