Solve: Double Up Null

Created By: Chris Taylor

/ Version: 2023.6.0

Write a method doubleUpNull that accepts a list of Strings and returns a list of Strings where each String in the original list is concatenated with itself. If a String in the list is null, simply pass it through, as shown below. If the passed List is null, return null. Your solution must make use of a stream and at least one lambda expression and/or method reference.

• ["a", "b"] -> ["aa", "bb"]
• ["", null] -> ["", null]
• null -> null