fix: allow maxMultiplexForwards to be set to zero

[ldeffenb]

Checklist

• [ ] I have read the coding guide.
• [ ] My change requires a documentation update, and I have done it.
• [ ] I have added tests to cover my changes.
• [ ] I have filled out the description and linked the related issues.

Description

If maxMultipleForwards is set to zero, pushsync is completely disabled. This is not good.

Open API Spec Version Changes (if applicable)

Motivation and Context (Optional)

Ensure that future coders don't run into this unintended issue if they set this to zero.

Related Issue (Optional)

#4681

Screenshots (if appropriate):