Use field.getUnlessDefault instead of field.get in UrlFormDataEncoder

[bpholt]

Fixes #1321.

PR Checklist (not all items are relevant to all PRs)

• Added unit-tests (for runtime code)
• Added bootstrapped code + smoke tests (when the rendering logic is modified)
• Added build-plugins integration tests (when reflection loading is required at codegen-time)
• Added alloy compliance tests (when simpleRestJson protocol behaviour is expanded/updated)
• Updated dynamic module to match generated-code behaviour
• Added documentation
• Updated changelog

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[bpholt]

@Baccata could you help me figure out where to add unit tests for this change? It's unclear to me where they belong—maybe there aren't any unit tests for UrlFormDataEncoder yet?

[Baccata]

https://github.com/disneystreaming/smithy4s/blob/series/0.18/modules/bootstrapped/test/src/smithy4s/http/internals/UrlFormDataEncoderDecoderSchemaVisitorSpec.scala

The unit tests of the core module live in the bootstrapped module, which makes it easier to write assertions against the generated code

[kubukoz]

Please add a CHANGELOG entry for 0.18.5. Other than that, looks good to me