typescript: Enable strictNullChecks

[krystofwoldrich]

These are enabled in JS Core since the last major v8.

Let's enable them in RN as well.

This PR should not introduce any behavior changes, only fix the null checks.

[github-actions]

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## Unreleased

- Enable strictNullChecks ([#4760](https://github.com/getsentry/sentry-react-native/pull/4760))

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Generated by 🚫 dangerJS against 3f3a70b

[github-actions]

Android (legacy) Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	419.04 ms	423.62 ms	4.58 ms
Size	17.75 MiB	19.54 MiB	1.79 MiB

Baseline results on branch: v7

Startup times

Revision	Plain	With Sentry	Diff
9da5c4e	478.08 ms	467.46 ms	-10.63 ms
9fbcfaf	420.06 ms	435.31 ms	15.25 ms

App size

Revision	Plain	With Sentry	Diff
9da5c4e	17.75 MiB	20.16 MiB	2.41 MiB
9fbcfaf	17.75 MiB	19.54 MiB	1.79 MiB

[github-actions]

Android (new) Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	398.92 ms	403.75 ms	4.83 ms
Size	7.15 MiB	8.29 MiB	1.14 MiB

Baseline results on branch: v7

Startup times

Revision	Plain	With Sentry	Diff
9fbcfaf+dirty	417.92 ms	431.81 ms	13.90 ms
9da5c4e+dirty	399.70 ms	407.34 ms	7.64 ms

App size

Revision	Plain	With Sentry	Diff
9fbcfaf+dirty	7.15 MiB	8.29 MiB	1.14 MiB
9da5c4e+dirty	7.15 MiB	8.42 MiB	1.27 MiB

[github-actions]

iOS (new) Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	1217.24 ms	1216.37 ms	-0.88 ms
Size	3.19 MiB	4.34 MiB	1.15 MiB

Baseline results on branch: v7

Startup times

Revision	Plain	With Sentry	Diff
9fbcfaf+dirty	1215.22 ms	1222.26 ms	7.04 ms
9da5c4e+dirty	1215.41 ms	1226.38 ms	10.97 ms

App size

Revision	Plain	With Sentry	Diff
9fbcfaf+dirty	3.19 MiB	4.34 MiB	1.15 MiB
9da5c4e+dirty	3.19 MiB	4.33 MiB	1.14 MiB

[github-actions]

iOS (legacy) Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	1223.60 ms	1236.98 ms	13.38 ms
Size	2.63 MiB	3.77 MiB	1.14 MiB

Baseline results on branch: v7

Startup times

Revision	Plain	With Sentry	Diff
9fbcfaf+dirty	1216.66 ms	1212.06 ms	-4.60 ms
9da5c4e+dirty	1231.84 ms	1235.49 ms	3.65 ms

App size

Revision	Plain	With Sentry	Diff
9fbcfaf+dirty	2.63 MiB	3.77 MiB	1.14 MiB
9da5c4e+dirty	2.63 MiB	3.76 MiB	1.13 MiB