Fix using strings in update_on of lilac.yaml

[yan12125]

Previously, when lilac.yaml contains the following configuration

update_on:
  - vcs

The generated nvchecker.ini contains the following section

[foobar]
vcs

And results in an error

Traceback (most recent call last):
  File "/usr/lib/python3.7/site-packages/nvchecker/core.py", line 152, in worker
    ret = await get_version(name, conf, keyman=self.keymanager)
  File "/usr/lib/python3.7/site-packages/nvchecker/get_version.py", line 62, in get_version
    version = await func(name, conf, **kwargs)
  File "/usr/lib/python3.7/site-packages/nvchecker/source/vcs.py", line 40, in get_version
    stderr=asyncio.subprocess.PIPE,
  File "/usr/lib/python3.7/asyncio/subprocess.py", line 217, in create_subprocess_exec
    stderr=stderr, **kwds)
  File "/usr/lib/python3.7/asyncio/base_events.py", line 1505, in subprocess_exec
    f"program arguments must be a bytes or text string, "
TypeError: program arguments must be a bytes or text string, not NoneType

The correct nvchecker configuration should be

[foobar]
vcs=

So I use an empty string instead