optional fields: default to null

[derhuerst]

Fixes #20. Closes #21.

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On my slow computer (Node.js v14.9.0) with e24d84f:

JSON (baseline) encoding: x 460,411 ops/sec ±1.81% (595 runs sampled)
JSON (baseline) decoding: x 449,983 ops/sec ±0.57% (597 runs sampled)
JSON (baseline) encoding + decoding: x 221,873 ops/sec ±0.13% (598 runs sampled)
protocol-buffers encoding: x 734,263 ops/sec ±0.61% (597 runs sampled)
protocol-buffers decoding: x 1,132,320 ops/sec ±0.12% (596 runs sampled)
protocol-buffers encoding + decoding: x 391,797 ops/sec ±0.08% (597 runs sampled)

without e24d84f:

JSON (baseline) encoding: x 499,192 ops/sec ±0.16% (597 runs sampled)
JSON (baseline) decoding: x 458,396 ops/sec ±0.11% (597 runs sampled)
JSON (baseline) encoding + decoding: x 225,796 ops/sec ±0.12% (597 runs sampled)
protocol-buffers encoding: x 727,112 ops/sec ±0.08% (597 runs sampled)
protocol-buffers decoding: x 1,050,047 ops/sec ±0.21% (597 runs sampled)
protocol-buffers encoding + decoding: x 402,329 ops/sec ±0.07% (597 runs sampled)