Handle present-but-empty extension data when serializing AuthenticatorData #391

Job	Run time
build (ubuntu-latest, x86_64-unknown-linux-gnu, true, sudo apt-get update sudo apt-get install -y...	1m 13s
build (ubuntu-latest, x86_64-unknown-linux-gnu, true, sudo apt-get update sudo apt-get install -y...	7m 22s
build (ubuntu-latest, x86_64-unknown-linux-gnu, true, sudo apt-get update sudo apt-get install -y...	2m 3s
build (macos-latest, x86_64-apple-darwin, true, --features crypto_dummy --no-default-features --t...	2m 31s
build (windows-latest, x86_64-pc-windows-gnu, true, --features crypto_dummy --no-default-features...	2m 14s
build (ubuntu-latest, x86_64-unknown-freebsd, false, sudo apt-get update sudo apt-get install -y ...	1m 43s
build (ubuntu-latest, x86_64-unknown-netbsd, false, sudo apt-get update sudo apt-get install -y l...	1m 39s
build (ubuntu-latest, x86_64-linux-android, false, sudo apt-get update sudo apt-get install -y li...	1m 5s
	19m 50s

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