Add the default value for the 'servers' array

The specification:

https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.0.1.md#oasObject

requires that if the 'servers' array is not provided or empty,
its default value is an array of a single Server Object
with 'url' of '/'.

Author: pbasista

openapi_core/servers.py

@@ -41,6 +41,9 @@ def __init__(self, dereferencer):
 
     def generate(self, servers_spec):
         servers_deref = self.dereferencer.dereference(servers_spec)
+        if not servers_deref:
+            yield Server('/')
+            return
         for server_spec in servers_deref:
             url = server_spec['url']
             variables_spec = server_spec.get('variables', {})