solana-cli: shreds list falls back to all seats when no default keypair

[martinsander00]

Resolves: https://github.com/malbeclabs/infra/issues/998

Summary

• When no --all or -k flag is passed, shreds list defaults to filtering by the default keypair's public key
• Previously, if no default keypair was found, the command errored out
• Now it gracefully falls back to showing all seats instead of failing

Lines of Code

Section	Added	Removed
SDKs	+2	-3