fix(cache): Do not throw NPE when reading ANR marker

[romtsn]

📜 Description

FileUtils.readText can return null if file does not exist

💡 Motivation and Context

Closes getsentry/sentry-dart#3412

💚 How did you test it?

existing tests should cover

📝 Checklist

• I added GH Issue ID & Linear ID
• I added tests to verify the changes.
• No new PII added or SDK only sends newly added PII if sendDefaultPII is enabled.
• I updated the docs if needed.
• I updated the wizard if needed.
• Review from the native team if needed.
• No breaking change or entry added to the changelog.
• No breaking change for hybrid SDKs or communicated to hybrid SDKs.

🔮 Next steps

[buenaflor]

thanks! 👍

[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	297.92 ms	307.26 ms	9.34 ms
Size	1.58 MiB	2.13 MiB	559.54 KiB

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
33a08cc	267.08 ms	340.45 ms	73.37 ms
fc5ccaf	256.80 ms	322.36 ms	65.56 ms
fc5ccaf	276.52 ms	370.46 ms	93.93 ms
ee747ae	386.94 ms	431.43 ms	44.49 ms
fcec2f2	311.35 ms	384.94 ms	73.59 ms
cf708bd	408.35 ms	458.98 ms	50.63 ms
b3d8889	420.46 ms	453.71 ms	33.26 ms
ed33deb	337.52 ms	484.06 ms	146.54 ms
dba088c	365.46 ms	366.31 ms	0.85 ms
9fbb112	361.43 ms	427.57 ms	66.14 ms

App size

Revision	Plain	With Sentry	Diff
33a08cc	1.58 MiB	2.12 MiB	555.28 KiB
fc5ccaf	1.58 MiB	2.13 MiB	557.54 KiB
fc5ccaf	1.58 MiB	2.13 MiB	557.54 KiB
ee747ae	1.58 MiB	2.10 MiB	530.95 KiB
fcec2f2	1.58 MiB	2.12 MiB	551.51 KiB
cf708bd	1.58 MiB	2.11 MiB	539.71 KiB
b3d8889	1.58 MiB	2.10 MiB	535.07 KiB
ed33deb	1.58 MiB	2.13 MiB	559.52 KiB
dba088c	1.58 MiB	2.13 MiB	558.99 KiB
9fbb112	1.58 MiB	2.11 MiB	539.18 KiB

Previous results on branch: rz/fix/npe-anr-marker

Startup times

Revision	Plain	With Sentry	Diff
089c8d4	287.65 ms	397.78 ms	110.12 ms

App size

Revision	Plain	With Sentry	Diff
089c8d4	1.58 MiB	2.13 MiB	559.54 KiB