message: Fix types in preparation for changing jspb submessage return types to include `undefined`.
diff: @@ -86,7 +86,7 @@ class AesCtrHmacAeadKeyFactory implements KeyManager.KeyFactory {
  * Checks the parameters and size of a given keyFormat.
  *
  */
  - validateAesCtrKeyFormat(keyFormat: null|PbAesCtrKeyFormat):
  + validateAesCtrKeyFormat(keyFormat: undefined|null|PbAesCtrKeyFormat):
  {aesCtrParams: PbAesCtrParams, aesCtrKeySize: number, ivSize: number} {
  if (!keyFormat) {
  throw new SecurityException(
  @@ -113,7 +113,7 @@ class AesCtrHmacAeadKeyFactory implements KeyManager.KeyFactory {
  * Checks the parameters and size of a given keyFormat.
  *
  */
  - validateHmacKeyFormat(keyFormat: null|PbHmacKeyFormat): {
  + validateHmacKeyFormat(keyFormat: undefined|null|PbHmacKeyFormat): {
  hmacParams: PbHmacParams,
  hmacKeySize: number,
  hashType: string,
  @@ -256,7 +256,7 @@ export class AesCtrHmacAeadKeyManager implements KeyManager.KeyManager<Aead> {
  * Checks the parameters and size of a given AES-CTR key.
  *
  */
  - private validateAesCtrKey(key: null|PbAesCtrKey):
  + private validateAesCtrKey(key: undefined|null|PbAesCtrKey):
  {aesCtrKeyValue: Uint8Array, ivSize: number} {
  if (!key) {
  throw new SecurityException(
  @@ -274,7 +274,7 @@ export class AesCtrHmacAeadKeyManager implements KeyManager.KeyManager<Aead> {
  * Checks the parameters and size of a given HMAC key.
  *
  */
  - private validateHmacKey(key: null|PbHmacKey):
  + private validateHmacKey(key: undefined|null|PbHmacKey):
  {hmacKeyValue: Uint8Array, hashType: string, tagSize: number} {
  if (!key) {
  throw new SecurityException(
  