Internal security checks, how to reach 100 % code coverage?

I have a lot of classes similar to the following one:

public class Foo
{
    private readonly Ba _ba;

    private Foo(Ba ba)
    {
        if (ba is null) throw new ArgumentNullException(ba);

        _ba = ba;
    }
}

In other classes' internals, I call this constructor of Foo, but as this would be unintended, in each constructor call ba is not null.

I wrote a lot of test methods for the consisting framework, but I am unable to reach the 100 % of code coverage as the exception in the above code snippet is newer thrown.

I see the following alternatives:

• Remove the null check: This would work for the current project implementation, but whenever I might add an accidental call Foo(null), debugging will be more difficult.
• Decorate the constructor with [ExcludeFromCodeCoverage]: This would work for the current Foo(Ba) implementation, but whenever I might change the implementation, new code paths in the constructor could develope and accidentally be missed to test.

How would you solve the dilemma?

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Notes

The code example is written in C#, but the question might address a general unit testing/exception handling problem.

C# 8 might solve this problem by introducing non-nullable reference types, but I am searching for a good solution until it has been released stable.