How to make if statement with null parameter?

How do you make an if statement check if something is null, and will return null if the if statement is true?

public String getMiddle(String word)
   {
      // I don't know if 'is null' or 'return null' are actually things
      if (is null){
         return null;
      }
   }

Also, what would be an example of an input that would make it null?