Fix warning for unwrap_or_else in generated builders (#3307)

While attempting to upgrade to Smithy 1.42, I noticed a warning in the
codegen-client integration tests:
```
warning: unnecessary closure used to substitute value for `Option::None`
   --> json_rpc10/rust-client-codegen/src/operation/operation_with_defaults/_operation_with_defaults_output.rs:477:31
    |
477 |             default_int_enum: self.default_int_enum.unwrap_or_else(|| 1),
    |                               ^^^^^^^^^^^^^^^^^^^^^^--------------------
    |                                                     |
    |                                                     help: use `unwrap_or(..)` instead: `unwrap_or(1)`
    |
    = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unnecessary_lazy_evaluations
```

This change fixes these warnings by switching between unwrap_or_else and
unwrap_or based on the shape.

----

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