Fix Serde nullability annotations

[slinkydeveloper]

Fix #357

• Bump to JSpecify 1.0
• Fix Serde nullability annotations

[github-actions]

Test Results

 50 files  ±0   50 suites  ±0   7m 37s ⏱️ -7s
759 tests +1  752 ✅ +1  7 💤 ±0  0 ❌ ±0 
760 runs  ±0  753 ✅ ±0  7 💤 ±0  0 ❌ ±0 

Results for commit cbdf5d9. ± Comparison against base commit 4455eec.

This pull request removes 6 and adds 7 tests. Note that renamed tests count towards both.

dev.restate.sdk.TestSerdesTest ‑ Byte: 94
dev.restate.sdk.TestSerdesTest ‑ Double: 0.23987722093183717
dev.restate.sdk.TestSerdesTest ‑ Float: 0.30495483
dev.restate.sdk.TestSerdesTest ‑ Integer: 437733994
dev.restate.sdk.TestSerdesTest ‑ Long: -7309916266831488259
dev.restate.sdk.TestSerdesTest ‑ Short: -7334

dev.restate.sdk.TestSerdesTest ‑ Byte: -43
dev.restate.sdk.TestSerdesTest ‑ Double: 0.31112775963416117
dev.restate.sdk.TestSerdesTest ‑ Float: 0.15802449
dev.restate.sdk.TestSerdesTest ‑ Integer: -1992689252
dev.restate.sdk.TestSerdesTest ‑ Long: -3176588736170124200
dev.restate.sdk.TestSerdesTest ‑ Short: 1097
dev.restate.sdk.serde.jackson.JacksonSerdesTest ‑ [Till, Francesco]

♻️ This comment has been updated with latest results.