fix(NODE-5225): eagerly clear MongoClient.topology in MongoClient.close()

[alecgibson]

Description

At the moment, calling MongoClient.close() in quick succession leads to an error:

TypeError: Cannot read properties of undefined (reading 'close')
  at node-mongodb-native/src/mongo_client.ts:530:16
  at new Promise (<anonymous>)
  at LegacyMongoClient.close (src/mongo_client.ts:529:11)
  at processTicksAndRejections (node:internal/process/task_queues:95:5)
  at async Promise.all (index 1)
  at async Context.<anonymous> (test/integration/node-specific/mongo_client.test.ts:393:5)

This happens because:

1. The first call attempts to asynchronously close the session pool sessions
2. While this is happening, the second call passes the topology null check (since this.topology hasn't yet been unset, and tries to also close session pool sessions
3. Both calls then set const topology = this.topology but the second call will get undefined since the first call unset this.topology
4. The second call now tries to call close() on an undefined topology and throws

What is changing?

This change fixes the issue by moving the topology unset immediately after the null check, so we know it's always correctly set.

Is there new documentation needed for these changes?

No

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