ref: Make sentry_fromIso8601String nullable

[philipphofmann]

sentry_fromIso8601String can return nil, if passing in a garbage string. Now, the method signature correctly reflects this.

This came up while investigating #4582.

#skip-changelog

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.338%. Comparing base (3e98256) to head (ffce24e).
Report is 4 commits behind head on main.

Additional details and impacted files

@@              Coverage Diff              @@
##              main     #4982       +/-   ##
=============================================
- Coverage   92.440%   92.338%   -0.102%     
=============================================
  Files          669       670        +1     
  Lines        79250     79258        +8     
  Branches     28651     27812      -839     
=============================================
- Hits         73259     73186       -73     
- Misses        5894      5979       +85     
+ Partials        97        93        -4     

Files with missing lines	Coverage Δ
Sources/Sentry/SentryDateUtils.m	100.000% <ø> (ø)
...ests/SentryTests/Helper/SentryDateUtilsTests.swift	100.000% <100.000%> (ø)
...ests/Protocol/Codable/SentryDateCodableTests.swift	100.000% <100.000%> (ø)

... and 27 files with indirect coverage changes

---

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[github-actions]

Performance metrics 🚀

	Plain	With Sentry	Diff
Startup time	1215.90 ms	1231.88 ms	15.98 ms
Size	22.30 KiB	829.54 KiB	807.24 KiB

Baseline results on branch: main

Startup times

Revision	Plain	With Sentry	Diff
5f8ee7a	1253.96 ms	1264.98 ms	11.02 ms
32e64d1	1249.42 ms	1264.76 ms	15.34 ms
f5a9f3f	1238.20 ms	1246.65 ms	8.45 ms
699d76f	1221.63 ms	1249.92 ms	28.28 ms
2af280d	1219.33 ms	1239.98 ms	20.65 ms
5ea7b5a	1215.67 ms	1240.86 ms	25.19 ms
a532b18	1238.78 ms	1253.60 ms	14.82 ms
fd6a31c	1204.73 ms	1222.34 ms	17.61 ms
be2977c	1202.51 ms	1221.32 ms	18.81 ms
2384e6d	1225.90 ms	1245.47 ms	19.57 ms

App size

Revision	Plain	With Sentry	Diff
5f8ee7a	22.85 KiB	411.93 KiB	389.08 KiB
32e64d1	20.76 KiB	433.18 KiB	412.42 KiB
f5a9f3f	22.31 KiB	775.15 KiB	752.83 KiB
699d76f	21.58 KiB	631.83 KiB	610.25 KiB
2af280d	20.76 KiB	435.22 KiB	414.46 KiB
5ea7b5a	22.31 KiB	821.35 KiB	799.04 KiB
a532b18	22.31 KiB	756.40 KiB	734.09 KiB
fd6a31c	20.76 KiB	436.50 KiB	415.74 KiB
be2977c	22.85 KiB	407.67 KiB	384.83 KiB
2384e6d	21.58 KiB	572.21 KiB	550.63 KiB

[philprime]

LGTM