Support null safety and fix cannot get twitter open graph tags#25

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WingCH wants to merge 9 commits intoyungzhu:masterfrom WingCH:master

+344-221

Commits on May 13, 2021

add facebookexternalhit, Bypassing the User-Agent whitelist to receive server-side-rendered (SSR) HTML
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update example
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update dependencies
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merge pr: yungzhu#13
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add example
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Commits on May 14, 2021

try use "User-Agent" "WhatsApp/2" and add lihkg example
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Commits on Dec 15, 2021