The tl;dr is if null is on either side of the comparison then you need to use an extra = sign (i.e. !== or ===).
The longer story is the != and == operators test equivalency and call the .equals methods on the operands. So, for example, if you have two String objects with the same set of characters, == will return true.
But null is a primitive so it doesn’t have a .equals method to call and therefore it doesn’t really have the ability to test for equivalency. So the warning is saying you should use the identity operators, the ones with the extra equal sign. If you have to String objects with the same set of characters, === will return false because they are two different objects. The identity operator tests to see if the two operands point to the same object in memory.
This is one of the many updates I need to make across all the DPs to reflect changes since 2.2 and there will be more when 2.3 is released.