Fix the type of Billboard.texture

[dumganhar]

Reported at https://forum.cocos.org/t/topic/165212/73?u=dumganhar

Changelog

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Continuous Integration

This pull request:

• needs automatic test cases check.

  Manual trigger with @cocos-robot run test cases afterward.

• does not change any runtime related code or build configuration

  If any reviewer thinks the CI checks are needed, please uncheck this option, then close and reopen the issue.

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Compatibility Check

This pull request:

• changes public API, and have ensured backward compatibility with deprecated features.
• affects platform compatibility, e.g. system version, browser version, platform sdk version, platform toolchain, language version, hardware compatibility etc.
• affects file structure of the build package or build configuration which requires user project upgrade.
• introduces breaking changes, please list all changes, affected features and the scope of violation.

[github-actions]

👍 Package size ⤵ -10 bytes, old: 5193082, new: 5193072

Interface Check Report

! WARNING this pull request has changed these public interfaces:

@@ -46390,10 +46390,10 @@
     export class Billboard extends Component {
         /**
          * @zh Billboard纹理。
          */
-        get texture(): null;
-        set texture(val: null);
+        get texture(): Texture2D | null;
+        set texture(val: Texture2D | null);
         /**
          * @zh 高度。
          */
         get height(): number;