cleanup: Fix nullability declarations

Cleanup imported from an internal Google migration process, courtesy
of Laura Harker.

Change-Id: I11de518eafe6008938589e5250bdcaf8151267e9

lib/cea/i_caption_decoder.js

@@ -22,7 +22,7 @@ shaka.cea.ICaptionDecoder = class {
    * @param {!Uint8Array} userDataSeiMessage
    * This is a User Data registered by Rec.ITU-T T.35 SEI message.
    * It is described in sections D.1.6 and D.2.6 of Rec. ITU-T H.264 (06/2019).
-   * @param {!number} pts PTS when this packet was received, in seconds.
+   * @param {number} pts PTS when this packet was received, in seconds.
    */
   extract(userDataSeiMessage, pts) {}
 
@@ -45,7 +45,7 @@ shaka.cea.ICaptionDecoder = class {
  * Parsed Cue.
  * @typedef {{
  *   cue: !shaka.text.Cue,
- *   stream: !string
+ *   stream: string
  * }}
  */
 shaka.cea.ICaptionDecoder.ClosedCaption;