fix: filter nil plugin conf triggered by etcd dir init (apache#5204)

apisix/plugin.lua

@@ -244,11 +244,14 @@ function _M.load(config)
         http_plugin_names = {}
         stream_plugin_names = {}
         local plugins_conf = config.value
-        for _, conf in ipairs(plugins_conf) do
-            if conf.stream then
-                core.table.insert(stream_plugin_names, conf.name)
-            else
-                core.table.insert(http_plugin_names, conf.name)
+        -- plugins_conf can be nil when another instance writes into etcd key "/apisix/plugins/"
+        if plugins_conf then
+            for _, conf in ipairs(plugins_conf) do
+                if conf.stream then
+                    core.table.insert(stream_plugin_names, conf.name)
+                else
+                    core.table.insert(http_plugin_names, conf.name)
+                end
             end
         end
     end

t/cli/test_admin.sh

@@ -208,3 +208,24 @@ if ! echo "$out" | grep "Admin API can only be used with etcd config_center"; th
 fi
 
 echo "passed: Admin API can only be used with etcd config_center"
+
+# disable Admin API and init plugins syncer
+echo '
+apisix:
+  enable_admin: false
+' > conf/config.yaml
+
+rm logs/error.log
+make init
+make run
+
+make init
+
+if grep -E "failed to fetch data from etcd" logs/error.log; then
+    echo "failed: should sync /apisix/plugins from etcd when disabling admin normal"
+    exit 1
+fi
+
+make stop
+
+echo "pass: sync /apisix/plugins from etcd when disabling admin successfully"