Normalize usage of encryptionKeyName

When tape is encrypted, we store a name of encryption key (e.g., r_validation_v1) to look up for encryption key value.

When tape is unencrypted the default value is different in several places:

• in the catalogue encryptionKeyName is stored as null
• cta-admin --json ta ls displays "-"
• XrdCtaTapeLs sets "-"
• in protobuf TapeLsItem it is optional string type
• cta-get-encryption-key expects "-"

We should decide which default value we will use and apply it to all sources.