##### SUMMARY
Running ansible with `--limit a,` or `--limit a,,b` or `--limit ,` leads to (when `-vvv` is added)
```
ERROR! Unexpected Exception, this is probably a bug: string index out of range
the full traceback was:

Traceback (most recent call last):
  File ".../ansible/bin/ansible-playbook", line 123, in <module>
    exit_code = cli.run()
  File ".../ansible/lib/ansible/cli/playbook.py", line 116, in run
    CLI.get_host_list(inventory, context.CLIARGS['subset'])
  File ".../ansible/lib/ansible/cli/__init__.py", line 487, in get_host_list
    inventory.subset(subset)
  File ".../ansible/lib/ansible/inventory/manager.py", line 614, in subset
    if x[0] == "@":
IndexError: string index out of range
ERROR! Unexpected Exception, this is probably a bug: string index out of range
to see the full traceback, use -vvv
```

##### ISSUE TYPE
- Bug Report

##### COMPONENT NAME
lib/ansible/inventory/manager.py

##### ANSIBLE VERSION
```
2.9.0
```
Also happens with `devel`, and potentially older ansible versions.

##### STEPS TO REPRODUCE
Run `ansible-playbook playbook.yml --limit a,` for an existing playbook `playbook.yml` (doesn't matter what it actually does, and also does not matter whether you use `a` or a real host name). Same result with `--limit a,,b` or similar expressions where `.split(',')` will lead to an empty string in the resulting list.

##### EXPECTED RESULTS
Ansible complaining about syntax, or simply ignoring the trailing comma / empty parts of the `.split(',')`.

##### ACTUAL RESULTS
```
ERROR! Unexpected Exception, this is probably a bug: string index out of range
to see the full traceback, use -vvv
```

"""
        Limits inventory results to a subset of inventory that matches a given
        pattern, such as to select a given geographic of numeric slice amongst
        a previous 'hosts' selection that only select roles, or vice versa.
        Corresponds to --limit parameter to ansible-playbook
        """
        if subset_pattern is None:
            self._subset = None
        else:
            subset_patterns = split_host_pattern(subset_pattern)
            results = []
            
            for x in subset_patterns:
                if x[0] == "@":
                    fd = open(x[1:])
                    results.extend([to_text(l.strip()) for l in fd.read().split("\n")])
                    fd.close()
                else:
                    results.append(to_text(x))
            self._subset = results
    def remove_restriction(self):
        """ Do not restrict list operations """
        self._restriction = None
    def clear_pattern_cache(self):
        self._pattern_cache = {}