[22.05] Check for app_name and link_name before unquoting

[mvdbeek]

Should fix
https://sentry.galaxyproject.org/share/issue/f20a1d47fab04ca8bca7a657d81fb568/:

AttributeError: 'NoneType' object has no attribute 'replace'
  File "/cvmfs/main.galaxyproject.org/venv/lib/python3.8/site-packages/paste/recursive.py", line 85, in __call__
    return self.application(environ, start_response)
  File "galaxy/web/framework/middleware/statsd.py", line 29, in __call__
    req = self.application(environ, start_response)
  File "/cvmfs/main.galaxyproject.org/venv/lib/python3.8/site-packages/paste/httpexceptions.py", line 640, in __call__
    return self.application(environ, start_response)
  File "galaxy/web/framework/base.py", line 159, in __call__
    return self.handle_request(environ, start_response)
  File "galaxy/web/framework/base.py", line 244, in handle_request
    body = method(trans, **kwargs)
  File "galaxy/web/framework/decorators.py", line 138, in set_nocache_headers
    return func(self, trans, *args, **kwargs)
  File "galaxy/webapps/galaxy/controllers/dataset.py", line 738, in display_application
    link_name = unquote_plus(link_name)
  File "urllib/parse.py", line 777, in unquote_plus
    string = string.replace('+', ' ')

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• This is a refactoring of components with existing test coverage.
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  1. [add testing steps and prerequisites here if you didn't write automated tests covering all your changes]

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