fix: check nil before calling clearTimer

[vpavlin]

Description

When setting up RPC server failed and a clean up process started, Filter v2 protocol attempted to call clearTimer on uninitialized attribute which caused SIGSEGV: Illegal storage access. (Attempt to read from nil?)

Changes

• check if maintenanceTask is Nil before calling clearTimer

How to test

1. Run Waku node as described in bug: nwaku randomly fails to start #1866

 docker run --network host -it statusteam/nim-waku:v0.18.0 --relay=true --filter=true --lightpush=true --rpc-admin=true --rpc-port=8545 --rpc-address=0.0.0.0 --websocket-support=true --nodekey=ca18f11664fed73af38b11ffc3ccdbdd0738c475488dfab18a91f43a1f78711a

2. Run the same command again in separate terminal - you'll get SIGSEGV because it fails to setup RPC server (conflicting port) and the clean up hits this issue
3. Compile wakunode2 from this PR and run it instead - you will get the actual error log and the clean up runs normally

ERR 2023-08-01 20:35:07.556+02:00 5/7 Starting node and protocols failed     topics="wakunode main" tid=34662 file=wakunode2.nim:92 error="failed to start waku node: (98) Address already in use"

Issue

closes #1866

[SionoiS]

The 🦀 in me says to always use options.

nil values are stupid and I hate them.

[Ivansete-status]

LGTM! Thanks for it!