Fix HasValueX incorrectly returning true

[SmithPlatts]

Overview

As per issue #675, when a whitespace string is passed to the Values(params object[] items) or Values(IEnumerable<object?> items) constructors of any of the Values<> classes, the HasValueX property will return true, where the corresponding ValueX will be empty.

Primary changes

• Added tests to handle the known failure scenario.
• Moved this.HasValueX evaluation and assignment to after this.ValueX assignment.

Ancillary changes

• Updated .editorconfig to redefine new Roslyn analysers, ensuring that newer warnings don't fail the build.
• Added a helper method to Schema.NET.Test, AssertEx.Empty<T>(OneOrMany<T> collection), which addresses the instances where the compiler was implicitly using string for Assert.Empty(OneOrMany<string>), instead of treating the input as an enumerable.

[Turnerj]

Hey @SmithPlatts - thanks for this! I'll try and review it soon as what you described in this and corresponding issue make sense.