When Rake parses an argument string with no tasks, it sets the top-level
task as "default".  Prior to this commit, if no default task was defined
(for example, if an app was generated with `--skip-test` and didn't
define its own default task), `Rails::Command::RakeCommand` would raise
`UnrecognizedCommandError`, preventing Rake from displaying the task
list.

This commit changes `Rails::Command::RakeCommand` to let Rake handle the
"default" task.

Fixes #50700.