Merge bitcoin/bitcoin#33853: kernel: Allow null arguments for serialized data

a3ac59a431 ci: Enable experimental kernel stuff in ASan task (MarcoFalke)
5b89956eeb kernel: Allow null arguments for serialized data (TheCharlatan)

Pull request description:

  An empty span constructed from an empty vector may have a null data pointer depending on the implementation. Remove the BITCOINKERNEL_ARG_NONNULL requirement for these arguments and instead handle such null arguments in the implementation.

  Also cherry-picked from #33845 to show that CI task passing now.

ACKs for top commit:
  yuvicc:
    Code review ACK a3ac59a431
  maflcko:
    review ACK a3ac59a431 🥈
  laanwj:
    code review ACK a3ac59a431

Tree-SHA512: 629e463796f2f057df5be8e8981a45751c578ed0021be731c1d57fe849a539fe38b0a445914b0fc48f32f0408ad6d566984bd7f3a68797fcfdf1c6889e316a08

.github/workflows/ci.yml

@@ -468,7 +468,7 @@ jobs:
             file-env: './ci/test/00_setup_env_arm.sh'
             provider: 'gha'
 
-          - name: 'ASan + LSan + UBSan + integer, no depends, USDT'
+          - name: 'ASan + LSan + UBSan + integer'
             cirrus-runner: 'ghcr.io/cirruslabs/ubuntu-runner-amd64:24.04-md' # has to match container in ci/test/00_setup_env_native_asan.sh for tracing tools
             fallback-runner: 'ubuntu-24.04'
             timeout-minutes: 120