##### 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
```

try:
            if not pattern_str[0] == '~':
                pattern = re.compile(fnmatch.translate(pattern_str))
            else:
                pattern = re.compile(pattern_str[1:])
        except Exception:
            raise AnsibleError('Invalid host list pattern: %s' % pattern_str)
        
        results = []
        for item in items:
            if pattern.match(item):
                results.append(item)
        return results
    def get_hosts(self, pattern="all", ignore_limits=False, ignore_restrictions=False, order=None):
        """
        Takes a pattern or list of patterns and returns a list of matching
        inventory host names, taking into account any active restrictions
        or applied subsets
        """
        hosts = []
        
        if isinstance(pattern, list):
            pattern_list = pattern[:]
        else:
            pattern_list = [pattern]
        if pattern_list:
            if not ignore_limits and self._subset:
                pattern_list.extend(self._subset)