fix: config is optional

[abraemer]

Summary of changes

When loading an input file without config field, we replace the resulting undefined with an object that represents an empty config properly.

Context and reason for change

Since config is optional, we need to handle the case when it is not present in the opossum file

How can the changes be tested

Not directly since nothing uses getClassifications right now. However my next PR cause errors on opossum files without config section.