Support yamless start

[LenarBad]

Currently we can't just skip the yaml config parameter and start Stubby4j with no pre-configured stubs.

I prefer to configure stubs via HTTP and that's great that we have that possibility but I have to use a workaround to avoid creating that useless (for me) yaml config.

I have to run Stubby4j with the fake value like -d no.yaml

Yes, it complains that couldn't find the file but starts somehow.

This is a fix for that, now you can run it with no yaml config parameter

On startup you can see that it starts with no yaml config:

No YAML config provided

[codecov-io]

Codecov Report

Merging #104 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #104   +/-   ##
=======================================
  Coverage   99.19%   99.19%           
=======================================
  Files          33       33           
  Lines         997      997           
  Branches      172      172           
=======================================
  Hits          989      989           
  Misses          3        3           
  Partials        5        5

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[azagniotov]

@LenarBad Hello. Apologies, but it took me only two years to merge your PR (with minor tweaks that were needed as a result of my manual testing). I have staged version stubby4j v7.0.1 for release on Maven Central