Workaround for https://github.com/vapor/vapor/issues/3046#issuecomment-1892883744

URLEncodedFormSanitizingMiddlware.swift

/// A middlware that filters a request body of type `application/x-www-form-urlencoded`
/// removing any key/value pairs having a zero-length value.
///
/// This is a remediation for a design defect where the presence of any such null entries
/// will cause model decoding to fail. See https://github.com/vapor/vapor/issues/3046.
struct URLEncodedFormSanitizingMiddlware: AsyncMiddleware {
    func respond(to request: Request,
                 chainingTo next: AsyncResponder) async throws -> Response {
        // Read the entire body content, which might not yet be available.
        let bodyString: String?
        if let string = request.body.string {
            bodyString = string
        } else {
            var bodyBuffer = try await request.body.collect(upTo: Int.max)
            bodyString = bodyBuffer.readString(length: bodyBuffer.readableBytes)
        }
        
        guard request.content.contentType == .urlEncodedForm,
              let bodyString else {
            // Pass through unmodified.
            return try await next.respond(to: request)
        }
        
        // Strip out any null-value entries.
        let cleanedString = bodyString.replacing(try Regex("[A-Za-z0-9_]+=(&|$)"), with: "")
        
        // Contrive a replacement request in the image of the original.
        let newRequest = Request(application: request.application,
                                 method: request.method,
                                 url: request.url,
                                 version: request.version,
                                 headers: request.headers,
                                 collectedBody: ByteBuffer(string: cleanedString),
                                 remoteAddress: request.remoteAddress,
                                 logger: request.logger,
                                 byteBufferAllocator: request.byteBufferAllocator,
                                 on: request.eventLoop)
        return try await next.respond(to: newRequest)
    }
}