{Core} Add check for empty command_str in _search_in_extension_commands

[fengzhou-msft]

Description

In the case of running az --debug a, the command_str got with command_str = roughly_parse_command(args[1:]) is empty so the _search_in_extension_commands function will return all extensions with a prefix match. Users are then prompted to choose an extension to install from all extensions.

To fix the issue, this PR adds empty check for command_str to return None for empty value.

Testing Guide

Run az --debug a.

History Notes

[Component Name 1] BREAKING CHANGE: az command a: Make some customer-facing breaking change.
[Component Name 2] az command b: Add some customer-facing feature.

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