fix: operator throwing panic when storage is empty

[csatib02]

Overview

• I placed a check in vault_types.go so the panic won't occur.
• Added a check, which creates a new status and produces an error, if the specified spec lacks storage configurations.

Fixes #189

Notes for reviewer

• The test is quite limited, it specifically addresses the area that previously failed.

[akijakya]

LGTM, nice work, thanks for also adding a test! It would be nice if the Vault api would provide us a way to validate configs, but I couldn't find it so far. If we encounter more panics due to other missing config values, the code could be extended with a validation step in the future.