Windows authentication support

[jsbnr]

Description of the changes

Updated go-mssqldb package to latest and removed verification of username presence on arguments.

Windows authentication currently fails. Providing just the domain user as username and no password as specified in the docs and code causes an error such as this:
Error creating connection to SQL Server: InitialBytes InitializeSecurityContext failed

Upgrading the go-mssqldb package to latest alone to support NTLMv2 does not appear to solve the issue entirely as there appear to be a bug with providing username. We found that providing neither username nor password allows windows authentication to work correctly using the host. Therefore the hard validation on username is removed to allow connections to be made without wither username or password.

Relevant dependency issues:

• feature: support NTLMv2 denisenkom/go-mssqldb#146
• Need help formatting a connection string for Windows Auth denisenkom/go-mssqldb#450

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