TestApp.hardcoded_path is required even if not needed

[Res260]

TestApp.hardcoded_path is required in contentctl.yml even if we specify splunk_api_username and splunk_api_password to download the apps from splunkbase. Moreover, the serialization function for hardcoded_path serializes a None value as a 'None' of type str. I will open a PR to fix this.

[pyth0n1c]

Sorry for the delay in getting to this, as we have been helping out with some internal Splunk releases.

Thank you @Res260 , you have been hard at work with some AWESOME fixes and improvements. I admit that downloading apps from Splunkbase is a feature that used to work, but we have not used (or tested it) in quite some time. Thanks for exposing all these issue. I am going to merge all the following fixes/PRs down to a single new branch for evaluation.

Related Bugs:
#295
#319
#321

Related PRs:
#317
#320

I have opened the following PR which will be used to merge + test these changes together:
#322

I will leave these issues open until all of these are reviewed + merged and a new contentctl release it made!