Check sub-command for modernCLI only if specified

[apoorvdeshmukh]

This commit addresses #183.

git\go-sqlcmd\cmd\modern>.\sqlcmd.exe
1> select @@version
2> go



------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Microsoft SQL Server 2019 (RTM-GDR) (KB5014356) - 15.0.2095.3 (X64)
        Apr 29 2022 18:00:13
        Copyright (C) 2019 Microsoft Corporation
        Developer Edition (64-bit) on Windows 10 Enterprise 10.0 <X64> (Build 22621: ) (Hypervisor)


(1 row affected)