{Core} Fix --only-show-errors on root parser

[jiasli]

Description

Using --only-show-errors on root parser fails:

> az --only-show-errors
the following arguments are required: _command_package

Examples from AI knowledge base:
https://aka.ms/cli_ref
Read more about the command in reference docs

This PR fixes --only-show-errors on root parser, same as microsoft/knack#253.

This issue was found while investigating #20269.

[yonzhan]

Core