##### 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
```

subscripts = {
        'a': [('a', None), list(string.ascii_letters)],
        'a[0]': [('a', (0, None)), ['a']],
        'a[1]': [('a', (1, None)), ['b']],
        'a[2:3]': [('a', (2, 3)), ['c', 'd']],
        'a[-1]': [('a', (-1, None)), ['Z']],
        'a[-2]': [('a', (-2, None)), ['Y']],
        'a[48:]': [('a', (48, -1)), ['W', 'X', 'Y', 'Z']],
        'a[49:]': [('a', (49, -1)), ['X', 'Y', 'Z']],
        'a[1:]': [('a', (1, -1)), list(string.ascii_letters[1:])],
    }
    ranges_to_expand = {
        'a[1:2]': ['a1', 'a2'],
        'a[1:10:2]': ['a1', 'a3', 'a5', 'a7', 'a9'],
        'a[a:b]': ['aa', 'ab'],
        'a[a:i:3]': ['aa', 'ad', 'ag'],
        'a[a:b][c:d]': ['aac', 'aad', 'abc', 'abd'],
        'a[0:1][2:3]': ['a02', 'a03', 'a12', 'a13'],
        'a[a:b][2:3]': ['aa2', 'aa3', 'ab2', 'ab3'],
    }
    def setUp(self):
        fake_loader = DictDataLoader({})
        self.i = InventoryManager(loader=fake_loader, sources=[None])
    def test_split_patterns(self):
        for p in self.patterns:
            r = self.patterns[p]
            self.assertEqual(r, split_host_pattern(p))
        for p, r in self.pattern_lists:
            self.assertEqual(r, split_host_pattern(p))
    def test_ranges(self):