fix: check observer exists handleIncomingSubscriptionMessage

[pckilgore]

No Issue, although similar in nature to #515 #544 #596 (and PRs #609, #595).

Check observer exists before calling its methods in AppSyncRealTimeSubscriptionHandshakeLink##_handleIncomingSubscriptionMessage

This observer is being defaulted to null here, so I assume checking that and ignoring the event after logging is fine since I would expect the method to bail there rather than initialize to an invalid value? https://github.com/awslabs/aws-mobile-appsync-sdk-js/blob/fc173bfe3ce262b1dba422021fc57097c4926b7b/packages/aws-appsync-subscription-link/src/realtime-subscription-handshake-link.ts#L641

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[pckilgore]

Bumping. What can I do here to help get it merged?

[pckilgore]

Hello again. Can I do anything to assist this is being merged?

[Dollab]

It would be really really great if this could be merged 🙏

[simon-q]

I opened a PR that fixes this following the existing code. Please review it: https://github.com/awslabs/aws-mobile-appsync-sdk-js/pull/735