[configuration] fix error when loading empty policy chain

> src/apicast/configuration.lua:62: attempt to get length of local 'policies' (a userdata value)

gateway/src/apicast/configuration.lua

@@ -10,6 +10,7 @@ local next = next
 local lower = string.lower
 local insert = table.insert
 local setmetatable = setmetatable
+local null = ngx.null
 
 local re = require 'ngx.re'
 local env = require 'resty.env'
@@ -55,7 +56,7 @@ local function backend_endpoint(proxy)
 end
 
 local function build_policy_chain(policies)
-  if not policies then return nil, 'no policy chain' end
+  if not policies or policies == null then return nil, 'no policy chain' end
 
   local chain = {}
 

spec/configuration_spec.lua

@@ -58,6 +58,16 @@ describe('Configuration object', function()
       assert.equals(429, config.limits_exceeded_status)
     end)
 
+
+    describe('policy_chain', function()
+
+      it('works with null', function()
+        local config = configuration.parse_service({ proxy = { policy_chain = ngx.null }})
+
+        assert(config)
+      end)
+    end)
+
     describe('backend', function()
       it('defaults to fake backend', function()
         local config = configuration.parse_service({ proxy = {