Improve Gson deserialization handling (DEV) (#1715)

* Check whether the local config file exists before parsing it.

* Force types deserialized via Gson to be non-null.
Perform sanity checks and empty file deletion directly when trying to load the json file.

* Check that empty json file is deleted.

* LINTs

* Test parsing exception.

Co-authored-by: Kolya Opahle <k.opahle@sap.com>
Co-authored-by: harambasicluka <64483219+harambasicluka@users.noreply.github.com>
Co-authored-by: Ralf Gehrer <ralfgehrer@users.noreply.github.com>