Backend: cosmetic, use isNull #2511

Job	Run time
macOS--dotnet6-and-mono	8m 33s
macOS--mono	6m 47s
windows--legacyFramework	15m 34s
windows--dotnet6	8m 22s
linux22-github	4m 18s
linux22-github--newmono	4m 24s
linux22-vanilla--stockdotnet6	5m 29s
linux22-vanilla--stockmono	5m 14s
linux20-github	4m 39s
linux22-vanilla--newmono	8m 22s
linux22-vanilla--stockdotnet6-and-newmono	5s
linux22-vanilla--stockdotnet6-and-stockmono	8s
linux20-github--newmono	5s
linux20-vanilla--newmono	6s
linux20-vanilla--stockmono	9s
conventions	0s
snap_pkg	0s
	1h 12m 15s

Provide feedback