fix: more informative error (sigstore#1778)

When running the attach attestation command with empty file input the cli returns hard to understand output like:

```
file name too long
Using payload from: filename.json
Error: unexpected end of JSON input
main.go:46: error during command execution: unexpected end of JSON input
```

By some investigation I found out that attestation file was empty, yes this is a user-error, so I recon it could be more informative as it throws the error while parsing the input making the real error a bit shaded.

Signed-off-by: ybelMekk <youssef.bel.mekki@nav.no>

cmd/cosign/cli/attach/attestation.go

@@ -42,6 +42,10 @@ func AttestationCmd(ctx context.Context, regOpts options.RegistryOptions, signed
 		return err
 	}
 
+	if len(payload) == 0 {
+		return fmt.Errorf("%s payload is empty", signedPayload)
+	}
+
 	env := ssldsse.Envelope{}
 	if err := json.Unmarshal(payload, &env); err != nil {
 		return err