Check for document existence when checking startViewTransition

[TooTallNate]

My non-browser environment has window, but does not have document. Checking for document existence here makes React Router work for me.

[changeset-bot]

🦋 Changeset detected

Latest commit: 15e16d6

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 4 packages

Name	Type
react-router-dom	Patch
react-router	Patch
react-router-dom-v5-compat	Patch
react-router-native	Patch

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Click here if you're a maintainer who wants to add another changeset to this PR

[remix-cla-bot]

Hi @TooTallNate,

Welcome, and thank you for contributing to React Router!

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You may review the CLA and sign it by adding your name to contributors.yml.

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- The Remix team

[remix-cla-bot]

Thank you for signing the Contributor License Agreement. Let's get this merged! 🥳

[brophdawg11]

Oops - @TooTallNate sorry would you be able to rebase this and point it to dev?

[brophdawg11]

actually, scratch that - we're about to cut a release so I cherry-picked these commits into #11544 to be included.

[TooTallNate]

Oh whoops, sorry about that! Thanks for merging.