fix: nil pointer guard when no attestation is present

[migmartri]

Closes #184

Now the CLI shows there was an issue retrieving the attestation which was the original behavior, or "there is an attestation in progress"

$chainloop wf run describe --id 0c27d5d0-cc38-418c-a933-46b9ef02a532
┌───────────────────────────────────────────────────────┐
│ Workflow                                              │
├────────────────┬──────────────────────────────────────┤
│ ID             │ 548995cd-e64a-4f7f-bb6b-789f8fc07afb │
│ Name           │ test                                 │
│ Team           │                                      │
│ Project        │ baz                                  │
├────────────────┼──────────────────────────────────────┤
│ Workflow Run   │                                      │
├────────────────┼──────────────────────────────────────┤
│ ID             │ 0c27d5d0-cc38-418c-a933-46b9ef02a532 │
│ Initialized At │ 15 Jun 23 15:18 UTC                  │
│ Finished At    │ 15 Jun 23 15:18 UTC                  │
│ State          │ success                              │
│ Runner Link    │                                      │
└────────────────┴──────────────────────────────────────┘
WRN there was an issue retrieving the attestation