gradient agent list panics when the account has no agents

### Description

`doctl gradient agent list` panics instead of returning an empty list when the authenticated account has no Gradient AI agents.

### Steps to reproduce

1. Authenticate to an account that has no Gradient AI agents.
2. Run:

```sh
doctl gradient agent list --output json
```

### Actual behavior

The command panics:

```text
panic: runtime error: index out of range [0] with length 0

github.com/digitalocean/doctl/do.(*paginatedList).set(...)
do/pagination.go:41
github.com/digitalocean/doctl/do.PaginateResp(...)
do/pagination.go:68
github.com/digitalocean/doctl/do.(*gradientAIService).ListAgents(...)
do/gradientai.go:176
github.com/digitalocean/doctl/commands.RunAgentList(...)
commands/gradientai_agent.go:175
```

### Expected behavior

The command should return an empty JSON array (`[]`) and exit successfully.

### Environment

- doctl version: 1.164.0 (latest stable release)
- OS: macOS 26.5.2
- Architecture: arm64
- Installation method: Homebrew

The panic is reproducible with an active account and an otherwise successful API connection. No access token or account-specific identifiers are included in this report.

Start with do/pagination.go:41 and do/gradientai.go:176, then trace how commands/gradientai_agent.go:175 handles the agent list response. Reproduce with `doctl gradient agent list --output json` against an account with no agents, and verify the command exits successfully with an empty JSON array instead of panicking.

Written by the indexing model from the issue text.