[url_launcher] Migrates the url_launcher_web package to null safety

[mvanbeusekom]

Description

This pull request add null safety support to the url_launcher_web package.

This PR is currently in "draft" since several dependencies of the test project don't support null safety yet which results in to failing integration tests. As discussed with @ditman and @blasten we decided to leave the PR in draft until the packages the url_launcher_web depends on are also migrated (see also #3148 for the discussion).

The implementation depends on companion PR #3148 which add null safety support to the url_launcher package. Since PR #3148 isn't merged as of yet the dependency is currently based on a path reference, which should be fixed before merging this PR.

Related Issues

flutter/flutter#66395

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Breaking Change

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[creativecreatorormaybenot]

Resolves flutter/flutter#72239.

[blasten]

The nnbd branch has been merged into master. Feel free to reopen this PR and reassign it to me. Thanks!