Fix NRE in InProc store (#98)

Fixes #94

Author: duncansmart

src/SessionStateModule/SessionStateStoreProviderAsyncBase.cs

@@ -205,7 +205,7 @@ public override void Initialize(string name, NameValueCollection config)
             // So we have inserted this setting here and it will apply to all providers that this async module can use.
 
             // skipKeepAliveWhenUnused
-            var skipKA = config["skipKeepAliveWhenUnused"];
+            var skipKA = config?["skipKeepAliveWhenUnused"];
             if (skipKA != null && !bool.TryParse(skipKA, out _skipKeepAliveWhenUnused))
             {
                 throw new ConfigurationErrorsException(string.Format(SR.Invalid_provider_option, "skipKeepAliveWhenUnused", name));
@@ -215,4 +215,4 @@ public override void Initialize(string name, NameValueCollection config)
         private bool _skipKeepAliveWhenUnused = false;
         internal bool SkipKeepAliveWhenUnused { get { return _skipKeepAliveWhenUnused; } }
     }
-}
+}