Map schema to prevent undefined reference

[LukeWood]

I'm pretty sure this is a bug, but sometimes when I run the alpha of colyseus 0.16 with this version I get the error:

TypeError: Cannot read properties of undefined (reading 'Symbol($changes)')
    at [$filter] (/Users/lukewood/workspace/yeti/node_modules/.pnpm/@colyseus+schema@3.0.0-alpha.23/node_modules/@colyseus/schema/src/types/custom/MapSchema.ts:32:51)
    at Encoder.encode (/Users/lukewood/workspace/yeti/node_modules/.pnpm/@colyseus+schema@3.0.0-alpha.23/node_modules/@colyseus/schema/src/encoder/Encoder.ts:104:32)
    at Encoder.encodeView (/Users/lukewood/workspace/yeti/node_modules/.pnpm/@colyseus+schema@3.0.0-alpha.23/node_modules/@colyseus/schema/src/encoder/Encoder.ts:204:14)
    at SchemaSerializer.applyPatches (/Users/lukewood/workspace/yeti/node_modules/.pnpm/@colyseus+core@0.16.0-preview.31/node_modules/@colyseus/core/build/serializer/SchemaSerializer.js:93:78)
    at QuickplayRoom.broadcastPatch (/Users/lukewood/workspace/yeti/node_modules/.pnpm/@colyseus+core@0.16.0-preview.31/node_modules/@colyseus/core/build/Room.js:401:41)
    at Timeout.QuickplayRoom.handleTick [as _onTimeout] (/Users/lukewood/workspace/yeti/apps/bulletz/server/src/QuickplayRoom.ts:73:12)
    at listOnTimeout (node:internal/timers:573:17)
    at processTimers (node:internal/timers:514:7)

perhaps this is just a "workaround" rather than the fix of the root cause. I don't know the new repo structure that well, but hopefully this is a fix.