Allow empty `BackendResult` #4121

Job	Run time
Check tket library version	18s
Check C++ code formatting	0s
Build and test (tket)	0s
Check documentation build	0s
Build and test tket (macos-arm64)	0s
Build and test (tket)	1s
Build and test (tket) (ubuntu-22.04)	1s
Build and test (tket) (macos-12)	2s
Build and test (tket) (windows-2022)	2s
Build and test (tket)	0s
Build and test pytket (macos)	13m 12s
Build and test pytket (ubuntu)	9m 27s
Build and test pytket (windows)	21m 17s
Build and test pytket (macos-arm64)	2m 33s
Check pytket line and branch coverage	5s
Publish pytket coverage	0s
	46m 58s

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