Handle nil @mon_data in MonitorMixin#mon_synchronize #9654

Job	Run time
cargo clippy	37s
make (check, --enable-yjit=dev, --yjit-call-threshold=1 --yjit-verify-ctx --yjit-code-gc, -T --yj...	5m 14s
make (check, --enable-yjit=dev)	6m 2s
make (yjit-bindgen, To fix: use patch in logs, --with-gcc=clang-14 --enable-yjit=dev --enable-zji...	5m 11s
make (test-bundled-gems, --enable-yjit=dev)	15m 11s
cargo test	51s
make (check, RUSTC='rustc +1.58.0', 1.58.0)	4m 39s
YJIT Ubuntu result	2s
	37m 47s

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