fix(mine): handle exceptions when error response body is null in Regi…

…sterCanaryTask

Before this change the RetrofitError catch block would throw a NullPointerException when error response body is null at line no: https://github.com/spinnaker/orca/blob/613427f41bb16461a7ee8bb0f31212b438d75458/orca-mine/src/main/groovy/com/netflix/spinnaker/orca/mine/tasks/RegisterCanaryTask.groovy#L70

Here is the exact stack trace : java.lang.NullPointerException: Cannot set property 'status' on null object
	at org.codehaus.groovy.runtime.NullObject.setProperty(NullObject.java:80)
	at org.codehaus.groovy.runtime.InvokerHelper.setProperty(InvokerHelper.java:213)
	at org.codehaus.groovy.runtime.ScriptBytecodeAdapter.setProperty(ScriptBytecodeAdapter.java:496)
	at com.netflix.spinnaker.orca.mine.tasks.RegisterCanaryTask.execute(RegisterCanaryTask.groovy:70)

orca-mine/src/main/groovy/com/netflix/spinnaker/orca/mine/tasks/RegisterCanaryTask.groovy

@@ -62,7 +62,8 @@ class RegisterCanaryTask implements Task {
     } catch (RetrofitError re) {
       def response = [:]
       try {
-        response = re.getBodyAs(Map) as Map
+        def responseBody = re.getBodyAs(Map) as Map
+        response = responseBody!=null ? responseBody : response
       } catch (Exception e) {
         response.error = e.message
       }