Backend: handle null response from Eth server #3939

Job	Run time
linux-vanilla--newmono-only	3m 14s
linux-github--dotnet-and-mono	1m 7s
linux-github--dotnet-and-newmono	2m 12s
linux-vanilla--stockdotnet6-and-newmono	2m 1s
linux-vanilla--stockdotnet6-and-stockmono	2m 7s
linux-vanilla--stockdotnet6-only	1m 24s
linux-vanilla--stockmono-only	2m 12s
macOS--dotnet6-and-mono	6m 4s
macOS--dotnet6-only	1m 13s
macOS--mono-only	5m 26s
windows--dotnet6-and-legacyFramework	2m 56s
windows--dotnet6-only	2m 25s
windows--legacyFramework-only	10m 23s
conventions	0s
snap_pkg	0s
snap_pkg_beta	0s
	42m 44s

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