mem-garnet: Fix default value of network bridge

Initializing the network bridge with NULL causes it to have
an class error when instatiating a link. The bridge is only
needed whne either a CDC or SerDes is enabled. This is handled
later during construction of the GarnetLink.

Change-Id: If19a21a6d9bf49449b9c390467d08d3422ae991a
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34257
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
diff --git a/src/mem/ruby/network/garnet2.0/GarnetLink.py b/src/mem/ruby/network/garnet2.0/GarnetLink.py
index 33a3d31..9f7fe8b 100644
--- a/src/mem/ruby/network/garnet2.0/GarnetLink.py
+++ b/src/mem/ruby/network/garnet2.0/GarnetLink.py
@@ -140,14 +140,14 @@
     # The network bridge encapsulates both the CDC and Ser-Des
     # units in HeteroGarnet. This is automatically enabled when
     # either CDC or Ser-Des is enabled.
-    ext_net_bridge = VectorParam.NetworkBridge(NULL,
+    ext_net_bridge = VectorParam.NetworkBridge([],
         "Network Bridge at external end")
-    ext_cred_bridge = VectorParam.NetworkBridge(NULL,
+    ext_cred_bridge = VectorParam.NetworkBridge([],
         "Credit Bridge at external end")
-    int_net_bridge = VectorParam.NetworkBridge(NULL,
+    int_net_bridge = VectorParam.NetworkBridge([],
         "Network Bridge at internal end")
-    int_cred_bridge = VectorParam.NetworkBridge(NULL,
-        "Credit Bridge at intternal end")
+    int_cred_bridge = VectorParam.NetworkBridge([],
+        "Credit Bridge at internal end")
 
 
     width = Param.UInt32(Parent.ni_flit_size,