[minigraph]: Do not fail for minigraphs which do not have neighbors listed in <Devices> section (sonic-net#2522)

Signed-off-by: Nadiya.Stetskovych <[email protected]>

Author: NStetskovych-zz

files/build_templates/buffers_config.j2

@@ -44,7 +44,7 @@ def
     {%- set cable_len = [] %}
     {%- for local_port in DEVICE_NEIGHBOR %}
         {%- if local_port == port_name %}
-            {%- if DEVICE_NEIGHBOR_METADATA[DEVICE_NEIGHBOR[local_port].name] %}
+            {%- if DEVICE_NEIGHBOR_METADATA is defined and DEVICE_NEIGHBOR_METADATA[DEVICE_NEIGHBOR[local_port].name] %}
                 {%- set neighbor = DEVICE_NEIGHBOR_METADATA[DEVICE_NEIGHBOR[local_port].name] %}
                 {%- set neighbor_role = neighbor.type %}
                 {%- set roles1 = switch_role + '_' + neighbor_role %}