fix: backport map batch length to length in pipeline.js

[mgrunberg]

Would you consider backporting https://github.com/stipsan/ioredis-mock/pull/1201 to v7.x?

I'm using the lib and I'm not ready to upgrade to v8 yet.

[github-actions[bot]]

:tada: This PR is included in version 7.5.1 :tada:

The release is available on:

• npm package (@release-7.x dist-tag)
• GitHub release

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