fix(admin): allow to receive empty tags

Previously Admin API crashed when tags=''. This commit
treats tags='' as empty.

KAG-5496
Fix #13591

kong/api/endpoints.lua

@@ -155,7 +155,7 @@ local function extract_options(args, schema, context)
       args.ttl = nil
     end
 
-    if schema.fields.tags and args.tags ~= nil and context == "page" then
+    if schema.fields.tags and args.tags ~= nil and args.tags ~= null and context == "page" then
       local tags = args.tags
       if type(tags) == "table" then
         tags = tags[1]

spec/02-integration/04-admin_api/14-tags_spec.lua

@@ -36,6 +36,8 @@ describe("Admin API - tags", function()
           })
         end
 
+        bp.consumers:insert({ username = "utagged-consumer"})
+
         assert(helpers.start_kong {
           database = strategy,
         })
@@ -73,6 +75,16 @@ describe("Admin API - tags", function()
         end
       end)
 
+      it("filter by emtpy tag", function()
+        local res = assert(client:send {
+          method = "GET",
+          path = "/consumers?tags="
+        })
+        local body = assert.res_status(200, res)
+        local json = cjson.decode(body)
+        assert.equals(3, #json.data) -- both tagged and 1 untagged
+      end)
+
       it("filter by multiple tags with AND", function()
         local res = assert(client:send {
           method = "GET",