patch undefined method for nil on validator 500 response (#530)

* patch undefined method for nil on validator 500 response

* better code

lib/inferno/dsl/fhir_resource_validation.rb

@@ -285,7 +285,7 @@ def operation_outcome_from_hl7_wrapped_response(response_hash)
           end
 
           # assume for now that one resource -> one request
-          issues = response_hash['outcomes'][0]['issues']&.map do |i|
+          issues = (response_hash.dig('outcomes', 0, 'issues') || []).map do |i|
             { severity: i['level'].downcase, expression: i['location'], details: { text: i['message'] } }
           end
           # this is circuitous, ideally we would map this response directly to message_hashes