[21.09] Fix error getting toolshed repo metadata when no repo found

[simonbray]

Fixes

IndexError: list index out of range
  File "galaxy/web/framework/decorators.py", line 312, in decorator
    rval = func(self, trans, *args, **kwargs)
  File "tool_shed/webapp/api/repositories.py", line 383, in index
    all_metadata['current_changeset'] = repository_found[0]

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