Upstream: return error if the value given is cjson.null

On read JSON config the null values are not consider nil. Due to the
latest changes on porta related to the null api_backend, APIcast will
raise an exception and things started to fail.

Related-to: 3scale/porta#1385
Fix: THREESCALE-3869

Signed-off-by: Eloy Coto <eloy.coto@gmail.com>

CHANGELOG.md

@@ -7,11 +7,13 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 ## [Unreleased]
 
 - Fix issues when TLS is enabled in Lazy mode [#1135](https://github.com/3scale/APIcast/pull/1135), [THREESCALE-3713](https://issues.jboss.org/browse/THREESCALE-3713)
+- Fix exception if api_backend is null [THREESCALE-3869](https://issues.jboss.org/browse/THREESCALE-3869) [PR #1136](https://github.com/3scale/APIcast/pull/1136)
 
 ### Added
 
 - Return 404 back if the upstream is not defined [THREESCALE-3775](https://issues.jboss.org/browse/THREESCALE-3775) [PR #1129](https://github.com/3scale/APIcast/pull/1129)
 
+
 ## [Unreleased]
 
 ### Added

gateway/src/apicast/proxy.lua

@@ -187,7 +187,6 @@ function _M.get_upstream(service)
   if not service then
     return errors.service_not_found()
   end
-
   local upstream, err = Upstream.new(service.api_backend)
 
   if not upstream then

gateway/src/apicast/upstream.lua

@@ -15,6 +15,7 @@ local url_helper = require('resty.url_helper')
 
 local http_proxy = require('apicast.http_proxy')
 local format = string.format
+local cjson = require('cjson')
 
 local _M = {
 
@@ -34,6 +35,10 @@ local mt = {
 --- @treturn nil|string error when upstream can't be initialized
 --- @static
 function _M.new(url)
+    if not url or url == cjson.null then
+        return nil, 'Upstream cannot be null'
+    end
+
     local uri, err = url_helper.parse_url(url)
     if err then
         return nil, 'invalid upstream'

t/apicast-blackbox.t

@@ -203,3 +203,36 @@ GET /?user_key=value
 could not find upstream for service: 42
 --- no_error_log
 [error]
+
+=== TEST 6: api backend  is null andreturns 404
+The request url is correct in the mapping rules but api_backend is null 
+--- configuration
+{
+  "services": [
+    {
+      "id": 42,
+      "backend_version":  1,
+      "proxy": {
+        "api_backend": null,
+        "proxy_rules": [
+          { "pattern": "/", "http_method": "GET", "metric_system_name": "hits", "delta": 2 }
+        ]
+      }
+    }
+  ]
+}
+--- backend
+location /transactions/authrep.xml {
+    echo 'ok';
+}
+--- upstream
+location / {
+    echo 'path: $uri';
+}
+--- request
+GET /?user_key=value
+--- error_code: 404
+--- error_log
+could not find upstream for service: 42
+--- no_error_log
+[error]