This story doesn't match with my observation programming.
Failure to sanity-check a null almost never stemmed from someone deleting the non-null assertion intentionally. It stemmed from someone rewiring the control flow in such a way as to bypass the non-null assertion without intending to (such as early-returning because they think they can shortcut the computation, but the shortcut isn't actually valid if the argument in question is null).
The tests have value precisely because they aren't in the regular control flow of the unit under test and will fail when the code flow is changed and the author didn't realize the change modified the expected behavior of the function.
> If somebody removes that line they had a reason.
That reason will be one of two things:
(1) They replaced it with something they thought would handle the same functionality (along with possibly some other functionality) in a beter way, or
(2) The requirement changed.
> That reason will make them remove the test that makes it fail as well.
If it was #2, sure (and in a test-first workflow, they would take out the test first). In case #1, they won’t take out the test, and if their implementation is wrong, the test will let them know.
> Tests that exist to check for the presence of a single line of code are deadweight.
Unless you have a very weird framework, tests don’t check the existence of lines of code, they check the behavior of units. The fact that the first-pass implementation in the same iteration in which the test was added is trivial doesn’t mean the function isn’t part of the contract that needs verified, and doesn’t mean that no one will ever change the intended implementation to a less direct one.
It’s possible that the code was modified in a way that bypasses the null-check. It isn’t necessarily true that anybody removed code.
Unit tests that test the contract of a method (if you put in this argument, then you’ll get this return or side effect) are a good idea in both dynamic and static languages.
Whether it's a good or a bad decision is another matter and not something the test will help with.
Tests that exist to check for the presence of a single line of code are deadweight.