[BUG] [Dart] API Responses incorrectly use `.body` instead of `.bodyBytes` when checking emptiness, which can crash for non-ut8 data

Bug Report Checklist

• Have you provided a full/minimal spec to reproduce the issue?
• Have you validated the input using an OpenAPI validator (example)?
• Have you tested with the latest master to confirm the issue still exists?
• Have you searched for related issues/PRs?
• What's the actual output vs expected output?
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Description

openapi-generator version

7.3.0-snapshot

    if (_response.body.isNotEmpty && _response.statusCode != HttpStatus.noContent) {
      return await apiClient.deserializeAsync(
        await _decodeBodyBytes(_response),
        'OutputFormat',
      ) as OutputFormat;
    }

the .body method implicitly converts to utf-8, which will throw exceptions when the data isn't a utf-8 string. Instead, when checking for emptiness, .bodyBytes should be used instead.

Start with the generated Dart response-handling code shown in the issue and trace where the response body is checked for emptiness before deserialization. Verify the behavior with non-UTF-8 response data and identify the relevant generated-client test coverage. Done means empty-response detection no longer implicitly decodes non-UTF-8 bytes and existing API response behavior remains intact.