fix: Don't assume that all service bindings existed before

[ghostbuster91]

Background

• What do these changes do?

I changed model.expectShape to model.getShape to so that the code doesn't fail if a given service is not present in the old model which can happen in certain cases.

• Why are they important?

This is a fix for software.amazon.smithy.model.node.ExpectationNotMetException: Shape not found in model: a.b#c that occurs when a new service is being added that references an already existing operation which was also changed in some way.

Full stacktrace:

 at software.amazon.smithy.model.Model.lambda$expectShape$1(Model.java:713)
        at java.base/java.util.Optional.orElseThrow(Optional.java:403)
        at software.amazon.smithy.model.Model.expectShape(Model.java:713)
        at software.amazon.smithy.model.Model.expectShape(Model.java:729)
        at software.amazon.smithy.aws.traits.auth.diff.SigV4Migration.evaluate(SigV4Migration.java:86)
        at software.amazon.smithy.diff.ModelDiff$Builder.lambda$compare$2(ModelDiff.java:293)
        at java.base/java.util.stream.ReferencePipeline$7$1.accept(ReferencePipeline.java:273)
        at java.base/java.util.ArrayList$ArrayListSpliterator.forEachRemaining(ArrayList.java:1625)
        at java.base/java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:509)
        at java.base/java.util.stream.AbstractPipeline.wrapAndCopyInto(AbstractPipeline.java:499)
        at java.base/java.util.stream.ReduceOps$ReduceTask.doLeaf(ReduceOps.java:960)
        at java.base/java.util.stream.ReduceOps$ReduceTask.doLeaf(ReduceOps.java:934)
        at java.base/java.util.stream.AbstractTask.compute(AbstractTask.java:327)
        at java.base/java.util.concurrent.CountedCompleter.exec(CountedCompleter.java:754)
        at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:373)
        at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1182)
        at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1655)
        at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1622)
        at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:165)

Testing

• How did you test these changes?

I minimized the issue and then reproduced it in the unit test.

Links

• Links to additional context, if necessary
• Issue #, if applicable (see here for a list of keywords to use for linking issues)

SigV4Migration was introduced in #2245

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[gosar]

This looks good. Thanks for fixing. I've committed @kubukoz 's suggestion to use isServiceShape. Will wait for CI to run and merge later.

[gosar]

nit: add newline