<PackageReference Include="NUnit" Version="4.3.2" />

NUnit.Framework.Constraints.EmptyConstraint

public class EmptyConstraint : Constraint
EmptyConstraint tests a whether a string or collection is empty, postponing the decision about which test is applied until the type of the actual argument is known.
namespace NUnit.Framework.Constraints { public class EmptyConstraint : Constraint { public EmptyConstraint(); } }