##### 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
```

if isinstance(pattern, list):
        return list(itertools.chain(*map(split_host_pattern, pattern)))
    elif not isinstance(pattern, string_types):
        pattern = to_text(pattern, errors='surrogate_or_strict')
    
    
    if u',' in pattern:
        patterns = pattern.split(u',')
    
    
    else:
        try:
            (base, port) = parse_address(pattern, allow_ranges=True)
            patterns = [pattern]
        except Exception:
            
            
            
            patterns = re.findall(
                to_text(r'''(?:     # We want to match something comprising:
                        [^\s:\[\]]  # (anything other than whitespace or ':[]'
                        |           # ...or...
                        \[[^\]]*\]  # a single complete bracketed expression)
                    )+              # occurring once or more
                '''), pattern, re.X
            )
    return [p.strip() for p in patterns]
class InventoryManager(object):
    ''' Creates and manages inventory '''
    def __init__(self, loader, sources=None):