PocketMine-MP server crash with certain invalid JSON payloads in `LoginPacket` due to dependency vulnerability (again)

Summary

### Impact An attacker could crash PocketMine-MP by sending malformed JSON in `LoginPacket`. This happened due to the particular handling of NULL types in the json mapper which accepts NULL type values in typed arrays which PocketMine-MP did not expect. Code processing arrays in the JSON data could then crash due to unexpected `NULL` elements. ### Patches This problem was fixed in 5.3.1 and 4.23.1 by updating JsonMapper to include the following commit: pmmp/netresearch-jsonmapper@4f90e8dab1c9df331fad7d3d89823404e882668c An upstream patch for this issue was proposed via https://github.com/cweiske/jsonmapper/pull/211; however, as of 2024-05-15, the patch has not been accepted upstream due to debate about how to deal with the behavior. For now, a fork of JsonMapper is used by PocketMine-MP to workaround the issue. ### Workarounds A plugin may handle `DataPacketReceiveEvent` for `LoginPacket` and check that none of the input arrays contain `NULL` where it's not expected, but this is rather cumbersome. ### References Proposed upstream patch for a behavior change: https://github.com/cweiske/jsonmapper/pull/211