Fix bug where GPV is sent with no parameters

lib/session.coffee

@@ -917,6 +917,7 @@ generateGetRpcRequest = (sessionContext) ->
       if sessionContext.deviceData.attributes.has(path)
         parameterNames.push(path)
 
+    if parameterNames.length
       return {
         name: 'GetParameterValues'
         parameterNames: (p.join('.') for p in parameterNames)