fix the `no function clause matching in Regex.named_capture/3`

crashed when opts[:git] is nil

```
** (FunctionClauseError) no function clause matching in Regex.named_captures/3
    (elixir) lib/regex.ex:232: Regex.named_captures(~r/^git:\/\/github.com\/(?<gh>.*).git$/, nil, [])
    lib/mix/tasks/deps.star.ex:17: anonymous fn/1 in Mix.Tasks.Deps.Star.run/1
    (elixir) lib/enum.ex:977: Enum."-map/2-lc$^0/1-0-"/2
    lib/mix/tasks/deps.star.ex:16: Mix.Tasks.Deps.Star.run/1
    (mix) lib/mix/cli.ex:55: Mix.CLI.run_task/2
```