##### 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
```

patterns = {
        'a': ['a'],
        'a, b': ['a', 'b'],
        'a , b': ['a', 'b'],
        ' a,b ,c[1:2] ': ['a', 'b', 'c[1:2]'],
        '9a01:7f8:191:7701::9': ['9a01:7f8:191:7701::9'],
        '9a01:7f8:191:7701::9,9a01:7f8:191:7701::9': ['9a01:7f8:191:7701::9', '9a01:7f8:191:7701::9'],
        '9a01:7f8:191:7701::9,9a01:7f8:191:7701::9,foo': ['9a01:7f8:191:7701::9', '9a01:7f8:191:7701::9', 'foo'],
        'foo[1:2]': ['foo[1:2]'],
        'a::b': ['a::b'],
        'a:b': ['a', 'b'],
        ' a : b ': ['a', 'b'],
        'foo:bar:baz[1:2]': ['foo', 'bar', 'baz[1:2]'],
    }
    pattern_lists = [
        [['a'], ['a']],
        [['a', 'b'], ['a', 'b']],
        [['a, b'], ['a', 'b']],
        [['9a01:7f8:191:7701::9', '9a01:7f8:191:7701::9,foo'],
         ['9a01:7f8:191:7701::9', '9a01:7f8:191:7701::9', 'foo']]
    ]