Add strong nil checking on orchestrator and redress test coverage

[peterbroadhurst]

• Fixes Passing an invalid namespace when connecting websocket causes a crash #1116
  • I went the route of forcing nil checking on the Operator() method everywhere (apart from unit tests, where I added a MustOrchestrator)
• Addresses a couple of areas where test coverage seems to have slipped

After fix:

[codecov-commenter]

Codecov Report

Merging #1130 (36cfeaf) into main (f1b10e9) will increase coverage by 0.26%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1130      +/-   ##
==========================================
+ Coverage   99.34%   99.61%   +0.26%     
==========================================
  Files         303      303              
  Lines       19695    19701       +6     
==========================================
+ Hits        19566    19625      +59     
+ Misses        120       75      -45     
+ Partials        9        1       -8     

Impacted Files	Coverage Δ
internal/apiserver/server.go	99.07% <100.00%> (-0.93%)	⬇️
internal/namespace/manager.go	100.00% <100.00%> (ø)
internal/blockchain/ethereum/ethereum.go	100.00% <0.00%> (+0.33%)	⬆️
internal/events/aggregator_rewind.go	100.00% <0.00%> (+0.65%)	⬆️
internal/blockchain/fabric/fabric.go	100.00% <0.00%> (+0.78%)	⬆️
internal/events/aggregator.go	100.00% <0.00%> (+2.74%)	⬆️
internal/orchestrator/persistence_events.go	100.00% <0.00%> (+3.70%)	⬆️
internal/contracts/manager.go	100.00% <0.00%> (+4.16%)	⬆️
internal/definitions/handler_contracts.go	100.00% <0.00%> (+4.47%)	⬆️
... and 2 more

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