no keyring and identity panic

[shahzadlone]

1. Build defradb

go build -o /tmp/defradb-bin ./cmd/defradb

2. Start with local ACP and no keyring

/tmp/defradb-bin start --no-keyring --rootdir /tmp/defradb-repro --document-acp-type local

3. In another terminal, add a policy

/tmp/defradb-bin client --no-keyring --rootdir /tmp/defradb-repro \ acp document policy add -f examples/policy/dac_policy.yml \ -i e3b722906ee4e56368f581cd8b18ab0f48af1ea53e635e3f7b8acd076676f6ac

4. Add a schema linked to the policy (use the PolicyID from step 3)

/tmp/defradb-bin client --no-keyring --rootdir /tmp/defradb-repro \ schema add 'type Users @policy(id: "<PolicyID from step 3>", resource: "users") { name: String }'

5. Add a document WITH identity - this triggers the panic

/tmp/defradb-bin client --no-keyring --rootdir /tmp/defradb-repro \ collection add --name Users '{"name": "Secret"}' \ -i e3b722906ee4e56368f581cd8b18ab0f48af1ea53e635e3f7b8acd076676f6ac

Step 5 returns Error: 500: and the server logs show:

panic: runtime error: invalid memory address or nil pointer dereference

 -> github.com/sourcenetwork/defradb/internal/core/block.signBlock
 ->   internal/core/block/signing.go:73

The panic is at ident.PrivateKey().Type() - when --no-keyring is used, the identity has no private key, so
PrivateKey() returns nil. Adding a document without --identity (step 4, schema add) works fine because no
signing is attempted. The crash only happens when both --no-keyring and --identity are used together on a
write operation.