Syntax issue when generating Javascript client operation without parameters

[thibaultclem]

When generating Client JS for an operation without parameters, an unnecessary comma is added before the callback parameter:

self.logoutUser = function(, callback) {

You can see the error directly on the generating UserApi.js:
https://github.com/swagger-api/swagger-codegen/blob/master/samples/client/petstore/javascript/src/scripts/rest/api/UserApi.js#L233

I provided a PR: #1690

PS: @jfiala thanks for this useful JS generator ;)

[wing328]

@thibaultclem thanks for reporting the issue and the PR, which has been merged into master.