model bucketing: special case for smithy.api#Unit

[kuhe]

In an edge case, the only model assigned to the last bucket may be smithy.api#Unit.

This causes the code generator to think that models_N exists, but in fact only models_(N-1) exists because Unit does not generate anything. This causes a compilation error in the generated models index.

// smithy-typescript generated code
export * from "./models_0";
export * from "./models_1"; // file doesn't exist

This proposed fix is to ignore smithy.api#Unit when incrementing the bucketing counter.

testing

tested code generation against the edge case unreleased feature

[kstich]

Can use .getId().equals(UnitTypeTrait.UNIT) instead.

[jhecking]

We are still running into this problem even after updating to 0.15.0 which includes this fix. The problem just happens later. With 0.14.0, the problem happens as soon as we add one more structure to our models. With 0.15.0, it only happens when we add two more structures to our model. Could there be some other shape type that should also default to bucket 0 – same as Unit shapes?