Make sure we can create an empty template

The initial bug was for HOT, but this fixes the same issue
in CFN too.

1) all calls to template[section] should return {} and not
   None by default (hot was correct, but not cfn)
2) the validate needs to access the raw template "self.t" so
   make sure we have a similar logic there too.
3) when warning for zero resources, look for len(resoruces) == 0 not
   None.

Change-Id: Iee6e2abbf13173c17d92f47b8a7efe207c29a984
Closes-bug: #1339893
Merge "Make sure we can create an empty template"