fix sample_golden_config_db.j2 portchannel undefined (#15311)

Add portchannel check to sample_golden_config_db.j2 to resolve errors
for topos (specifically mx) that do not have portchannels for
golden_config_infra/test_config_reload_with_rendered_golden_config.py
when running `sonic-cfggen -d -t /tmp/golden_config_db.j2 > /etc/sonic/golden_config_db.json`

tests/golden_config_infra/templates/sample_golden_config_db.j2

@@ -1,7 +1,9 @@
 {% set portchannels= [] %}
-{% for pc, value in PORTCHANNEL.items() %}
-    {% set _ = portchannels.append(pc) %}
-{% endfor %}
+{% if PORTCHANNEL is defined %}
+    {% for pc, value in PORTCHANNEL.items() %}
+        {% set _ = portchannels.append(pc) %}
+    {% endfor %}
+{% endif %}
 
 {
     "NEW_FEATURE": {