fix: model bucketing edge case with resource shape

[atbreuer11]

Issue:
Similar to this issue, when a resource shape is the only model assigned to the last bucket, an export statement for the file is included in index.ts, yet the file is never created, as there is no codgen for resources.

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

Description of changes:

• Update SymbolVisitor to always place resource shapes in initial bucket
• Add unit test