php – What is the technical terminology for the practice of checking for null and edge cases in Object-Oriented programming?

I took a PHP test for NASA recently and thought I should have done better. I believe the issue is something I used to know about, but need a refresher in. I can’t remember what it is called though. It is the concept that you have to check for “NULL” and empty and edge-cases. NO, it’s not unit testing or anything like that. It is a proactive programming technique involving conditionals and equality tests.

Does anyone know what the web-searchable, hashtaggable, technical term for this technique is?

DISCLAIMER

I thought this question might be considered by some as off-topic, though it is a terminology question, so it should be acceptable and useful to many searching for the same thing and can easily be seen as on-topic. But, just in case, I created this room on the advice of a comment here: Are Career-Advice Questions On-Topic Here? because I have some questions about code challenges and software interview prep that I intend to ask after I get this terminology question solved.