fix: check if db exists in update_app_envs_before_open_db when starting a replica

[nanorth]

Handle the manual_compact.once.trigger_time parameter in app_envs provided by duplication. When a backup cluster creates an app via duplication, incomplete replicas may trigger manual compaction if the source app has this parameter configured. However, these replicas have not yet opened RocksDB successfully, which can lead to coredumps.

[acelyc111-bot]

Review: Check db exists before manual compact

Summary: Adds a null check for _db before calling start_manual_compact_if_needed in update_app_envs_before_open_db. Prevents crash when this function is called during replica startup before the DB is opened.

What's good:

• Targeted, minimal fix — exactly the kind of change you want for a crash fix
• Correct placement: the other calls in the same function (update_rocksdb_iteration_threshold, update_validate_partition_hash, etc.) likely don't need the DB, but manual compaction does

Style issue:

if(_db){  // missing spaces

Should be:

if (_db) {

Verdict: ✅ Approve — Fix is correct. The style nit is minor and can be addressed in a follow-up or during merge.