Improve error message on empty "mix do"

[whatyouhide]

Yucksies!

→ mix do
** (FunctionClauseError) no function clause matching in anonymous fn/1 in Mix.Tasks.Do.run/1    
    
    The following arguments were given to anonymous fn/1 in Mix.Tasks.Do.run/1:
    
        # 1
        []
    
    (mix 1.17.3) lib/mix/tasks/do.ex:61: anonymous fn/1 in Mix.Tasks.Do.run/1
    (elixir 1.17.3) lib/enum.ex:987: Enum."-each/2-lists^foreach/1-0-"/2
    (mix 1.17.3) lib/mix/task.ex:495: anonymous fn/3 in Mix.Task.run_task/5
    (mix 1.17.3) lib/mix/cli.ex:96: Mix.CLI.run_task/2
    /Users/andrea/.asdf/installs/elixir/1.17.3/bin/mix:2: (file)

[josevalim]

And please backport to v1.18.

[whatyouhide]

Backported ✅