Cloud-init fails if metadata local-hostname is null

If I run cloud-init with a config drive containing a metadata file which has local-hostname set to null, I get the following traceback:

Traceback (most recent call last):
  File "/usr/lib/python2.7/site-packages/cloudinit/sources/__init__.py", line 447, in find_source
    if s.get_data():
  File "/usr/lib/python2.7/site-packages/cloudinit/sources/__init__.py", line 132, in get_data
    self._get_standardized_metadata())
  File "/usr/lib/python2.7/site-packages/cloudinit/sources/__init__.py", line 110, in _get_standardized_metadata
    'local-hostname': self.get_hostname(),
  File "/usr/lib/python2.7/site-packages/cloudinit/sources/__init__.py", line 317, in get_hostname
    if util.is_ipv4(lhost):
  File "/usr/lib/python2.7/site-packages/cloudinit/util.py", line 544, in is_ipv4
    toks = instr.split('.')

Tested on CentOS 7.7, cloud-init 18.5.