doc: document that iv may be null when using createCipheriv()

This adds `null` to the supported types of the `iv` option when
using `crypto.createCipheriv()`.

PR-URL: #29684
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>

doc/api/crypto.md

@@ -1643,7 +1643,7 @@ changes:
 
 * `algorithm` {string}
 * `key` {string | Buffer | TypedArray | DataView | KeyObject}
-* `iv` {string | Buffer | TypedArray | DataView}
+* `iv` {string | Buffer | TypedArray | DataView | null}
 * `options` {Object} [`stream.transform` options][]
 * Returns: {Cipher}
 
@@ -1737,7 +1737,7 @@ changes:
 
 * `algorithm` {string}
 * `key` {string | Buffer | TypedArray | DataView}
-* `iv` {string | Buffer | TypedArray | DataView}
+* `iv` {string | Buffer | TypedArray | DataView | null}
 * `options` {Object} [`stream.transform` options][]
 * Returns: {Decipher}