Only run event systems if they have tangible work to do (#7728) #219

Job	Run time
ci	4m 35s
miri	8m 4s
check-bevy-internal-imports	4s
build (windows-latest)	11m 24s
build (ubuntu-latest)	4m 35s
check-missing-examples-in-docs	1m 3s
check-doc	12m 57s
build (macos-latest)	7m 25s
run-examples-on-windows-dx12	29m 25s
check-missing-features-in-docs	59s
markdownlint	1m 25s
check-compiles	5m 44s
msrv	2m 11s
build-wasm	2m 5s
	1h 31m 56s

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