fix: fallbacks to an empty object during "getFromString"

[artem-zakharchenko]

• Fixes an issue in the Core app when headers.values.expected is not defined sometimes.

[honzajavorek]

We were able to identify the root cause and I think we should compensate for it in the apiary.io core app. Although we could say that this PR supports the robustness principle, I think in this case gavel2html shouldn't be benevolent and should throw on unexpected input structure. This actually helps to catch mistakes early.

[artem-zakharchenko]

I would love not to introduce this handling, as it's too much for gavel2html. Would need to investigate the core problem.