##### 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
```

for s in self.subscripts:
            r = self.subscripts[s]
            self.assertEqual(r[0], self.i._split_subscript(s))
            self.assertEqual(
                r[1],
                self.i._apply_subscript(
                    list(string.ascii_letters),
                    r[0][1]
                )
            )
class TestInventoryPlugins(unittest.TestCase):
    def test_empty_inventory(self):
        inventory = self._get_inventory('')
        self.assertIn('all', inventory.groups)
        self.assertIn('ungrouped', inventory.groups)
        self.assertFalse(inventory.groups['all'].get_hosts())
        self.assertFalse(inventory.groups['ungrouped'].get_hosts())
    def test_ini(self):
        self._test_default_groups("""
            host1
            host2
            host3
            [servers]
            host3
            host4
            host5
            """)
    def test_ini_explicit_ungrouped(self):