bug: proposal update is blocking indexing

[bonustrack]

I did this hotfix because the proposal update event was blocking indexing in the API with this error:

{"level":50,"time":1721314857665,"component":"checkpoint","blockNumber":659288,"err":{"type":"TypeError","message":"Cannot read properties of undefined (reading 'execution_strategy')","stack":"TypeError: Cannot read properties of undefined (reading 'execution_strategy')\n    at Object.handleUpdate (/workspace/apps/api/dist/src/writer.js:362:89)\n    at process.processTicksAndRejections (node:internal/process/task_queues:95:5)\n    at async StarknetProvider.handleTx (/workspace/node_modules/@snapshot-labs/checkpoint/dist/src/providers/starknet/provider.js:174:29)\n    at async StarknetProvider.handleBlock (/workspace/node_modules/@snapshot-labs/checkpoint/dist/src/providers/starknet/provider.js:86:13)\n    at async StarknetProvider.processBlock (/workspace/node_modules/@snapshot-labs/checkpoint/dist/src/providers/starknet/provider.js:55:9)\n    at async Checkpoint.next (/workspace/node_modules/@snapshot-labs/checkpoint/dist/src/checkpoint.js:303:37)\n    at async Checkpoint.start (/workspace/node_modules/@snapshot-labs/checkpoint/dist/src/checkpoint.js:160:16)"},"msg":"error occurred during block processing"}

https://github.com/snapshot-labs/sx-monorepo/pull/502/files

This is something we should fix