This is not a blind assumption that you make, but one based on lots of evidence (for example you are aware of the testing procedures and QA processes of the CPU manufacture, and you are aware that millions of people and companies use the same CPU model as you and if there were bugs they would have detected them).

If you were to buy some CPU that someone shady made themselves in their basement, them you would probably not make the assumption that the CPU works.

A lot of times programmers make assumptions without properly verifying that the assumption is true, and that is when problems happen. for example, when working with a library function, the programmer might test that it works with certain types of inputs and then just assume that it also works with other types, without actually checking the documentation or verifying

They don't mean that you just silently ignore the null value, they mean that they would prefer to use a language that has proper optional types.

You are doing exactly that, assuming nothing. The number of times I've seem "meh, x can't be null here, just do nothing about it" (instead of some way of asserting that assumption, such as you provide) is staggering.