Ensure bson-kotlin handles possible NPE

[rozza]

If the user provides their own CodecRegistry it could return null from CodecRegistry#get(clazz)

JAVA-5477

[stIncMale]

I think we should not do this ad hoc check. Both of the CodecRegistry.get methods don't allow returning nulls, so when a custom CodecRegistry.get returns null, it violates its contract. There are three ways we can go about that:

1. [the current approach in the driver, though I can't guarantee that it is used consistently] Do nothing, assuming nulls can't be returned, but also not asserting that values are non-null, because we use assertions only for driver bugs, and here a bug may be in the application code.
2. [the approach proposed in this PR, but the PR applies it to a single place of many] Everywhere we call CodecRegistry.get, we check if the result is null, and do something with it. This approach is error-prone, and we should avoid it.
3. [the approach I propose] Wrap all application-provided CodecRegistry objects in CheckedCodecRegistry, which checks the return value of the get methods for null, and either converts it into the CodecConfigurationException with accordance to the contract, or throws an NPE, informing an application that it violated the contract at runtime.
   3.1. If we were just now designing this API, I would have suggested the "throw NPE from CheckedCodecRegistry" approach. However, since we already have it implemented, and there is a chance we sometimes may do the ad hoc checks somewhere in the driver, I propose to use the "convert null into the CodecConfigurationException" approach.
   3.2 Following are all the places where a user may specify a custom CodecRegistry:
   • MongoClientSettings.Builder.codecRegistry
   • MongoCollection.withCodecRegistry
   • CodecRegistries.fromRegistries
     3.3 We can optimize wrapping by omitting it when the instance given to us is of the concrete type we own and know users may not extend: ChildCodecRegistry, ProvidersCodecRegistry, CheckedCodecRegistry.

[rozza]

The adhoc check is needed to inform the user if they are able to encode / decode the Kotlin DataClass and all its properties (fields). If it were to return null it means that there is no Codec available in the registry for the type.

Note: CodecRegistry.getCodec here is an extension function to CodecRegistry and is only for the use within the DataClassCodec. The call to this.get is a call to codecRegistry#get which uses the users configured CodecRegistry.

[rozza]

Check was a false positive :)