[sda-db] NOT NULL string in key_hash column in sda.encryption_keys table

[kostas-kou]

Describe the bug
One can create an entry in the sda.encryption_keys table which is empty
Steps to reproduce
Run the sda-stack locally and insert a row in the table by just using psql

Expected behavior
Not allowed to add an empty key hash in the table

Additional context

Estimation of size: small

Estimation of priority: medium