I have faced such warnings many times in past so my suggestion will be to ignore many of your NetBeans warnings, as IDE doesn't know, how will you use your inputs or variables. It will just warn you about the best practices that you should follow while coding in IDE, which is not necessary the best option for your code.
If you are using the filter_input(), then you don't need to use the
isset() additionally. As the
filter_input() is already doing the job for you.
So my suggestion for you is that you should just remove the
isset() and your code will do the job for you. And in addition to that if you want to check if your value was set to apply your alternative values. Then you can use empty() function it will return
FALSE to you when your value returned by the
FALSE. So you can just negate your condition to match your case.