MongoDB adapter sets beginTransaction to undefined which is not a valid value

[cbratschi]

Describe the Bug

The code here sets beginTransaction to undefined if there is no replica set:

https://github.com/payloadcms/payload/blob/main/packages/db-mongodb/src/connect.ts#L48

However, undefined is not a valid value. Better use:

if (!client.options.replicaSet) {
      this.transactionOptions = false
      this.beginTransaction = async () => null
}

Where null is a supported return type.

Link to the code that reproduces this issue

https://github.com/cbratschi/payload

Reproduction Steps

See above.

Which area(s) are affected? (Select all that apply)

db-mongodb

Environment Info

Binaries:
  Node: 22.12.0
  npm: 10.9.0
  Yarn: 1.22.22
  pnpm: N/A
Relevant Packages:
  payload: 3.13.0
Operating System:
  Platform: darwin
  Arch: arm64
  Version: Darwin Kernel Version 24.1.0: Thu Oct 10 21:05:23 PDT 2024; root:xnu-11215.41.3~2/RELEASE_ARM64_T6031
  Available memory (MB): 65536
  Available CPU cores: 16