It depends on what you are encrypting. If you are encrypting many thing, you need a way of telling how long each thing is so you can seperate them once the ciphertext is decrypted.
If you're encrypting more than just a string, You'll need to add a NUL if you remove the length. You have to use one or the other (or some other alternative) in order to know where the string ends and the next thing in the plaintext start.
If the ciphertext only contains the string, you don't need either mechanism since the length of the string will be deduced by the length of the ciphertext.
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