Fixing in-hash error when a port is provided and user.js isnt

[quxo]

If one uses the current master branch, an error is obtained when a port is passed but user.js has the default value #f.

For example if one runs:

(start-marionette! #:port 55000)

then one gets the error

in-hash: contract violation
expected: hash?
given: #f

Now, this happend because in /marionette-lib/support/user.js the code (in-hash user.js) runs even when user.js is #f.
This pull request solved this bug.

[Bogdanp]

Thanks!