Handle bootstrapping of Poseidon when no environments

When starting Poseidon with an empty Nomad cluster, the list of existing environments is empty. In this case, however, our error handler threw an error, which was not correct.

Fixes CODEOCEAN-M7

lib/runner/strategy/poseidon.rb

@@ -27,12 +27,11 @@ class Runner::Strategy::Poseidon < Runner::Strategy
     case response.status
       when 200
         response_body = parse response
-        execution_environments = response_body[:executionEnvironments]
 
-        if execution_environments.nil?
+        if response_body.key? :executionEnvironments
           raise(Runner::Error::UnexpectedResponse.new("Could not get the list of execution environments in Poseidon, got response: #{response.as_json}"))
         else
-          execution_environments
+          response_body[:executionEnvironments] || []
         end
       else
         handle_error response