Convert fabric MTU to string for API call

Change-Id: I8cda30d1f26624533d6adda02158d605bf1e76e8
diff --git a/_modules/maasng.py b/_modules/maasng.py
index a96a4e4..c08d5db 100644
--- a/_modules/maasng.py
+++ b/_modules/maasng.py
@@ -1095,10 +1095,11 @@
     data = {
         "name": name,
         "dhcp_on": str(dhcp_on),
-        "mtu": mtu,
         "description": description,
         "primary_rack": list_racks()[primary_rack]['system_id'],
     }
+    if mtu:
+        data['mtu'] = str(mtu)
     vlan = str(vlan)
     maas = _create_maas_client()
     fabric_id = get_fabricid(fabric)