Fix nil pointer panic

[marefr]

Fixes the following panic:

2021-03-31 07:34:57	
	/home/marcus/go/src/github.com/grafana/devtools/pkg/streams/memorybus/memory_bus.go:83 +0x1dd
2021-03-31 07:34:57	
created by github.com/grafana/devtools/pkg/streams/memorybus.(*InMemoryBus).Start.func1
2021-03-31 07:34:57	
	/home/marcus/go/src/github.com/grafana/devtools/pkg/streams/memorybus/memory_bus.go:84 +0x51
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github.com/grafana/devtools/pkg/streams/memorybus.(*InMemoryBus).Start.func1.1(0xc0002ad840, 0xc000202000, 0xc0338b1080, 0xc01127687105e6d0, 0x490eac5df0, 0x10a41a0, 0xc00032c160)
2021-03-31 07:34:57	
	/home/marcus/go/src/github.com/grafana/devtools/pkg/streams/projections/engine.go:95 +0x1b4
2021-03-31 07:34:57	
github.com/grafana/devtools/pkg/streams/projections.(*StreamProjection).createSubscriber.func1(0xadc320, 0xc000202000, 0xc0338b1080)
2021-03-31 07:34:57	
	/home/marcus/go/src/github.com/grafana/devtools/pkg/streams/projections/partitioning.go:45 +0x170
2021-03-31 07:34:57	
github.com/grafana/devtools/pkg/streams/projections.(*partionedProjection).Run(0xc000295410, 0xc0338b1080, 0x1c, 0xc03950e900, 0x2)
2021-03-31 07:34:57	
	/home/marcus/go/src/github.com/grafana/devtools/pkg/streams/projections/projection.go:67 +0x35b
2021-03-31 07:34:57	
github.com/grafana/devtools/pkg/streams/projections.(*projection).callApply(0xc0002932c0, 0x95f5a0, 0xc03f594900, 0x9bd280, 0xc041176a80, 0x0, 0x0, 0x0)
2021-03-31 07:34:57	
	/usr/local/go/src/reflect/value.go:308 +0xa4
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reflect.Value.Call(0x984820, 0xc000298dc0, 0x13, 0xc0212495f0, 0x2, 0x2, 0x1, 0x2, 0xc000073c78)
2021-03-31 07:34:57	
	/usr/local/go/src/reflect/value.go:447 +0x461
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reflect.Value.call(0x984820, 0xc000298dc0, 0x13, 0xa23a30, 0x4, 0xc0212495f0, 0x2, 0x2, 0x2, 0xc0212495f0, ...)
2021-03-31 07:34:57	
	/home/marcus/go/src/github.com/grafana/devtools/pkg/githubstats/release_annotation.go:49 +0x93
2021-03-31 07:34:57	
github.com/grafana/devtools/pkg/githubstats.(*ReleaseAnnotationProjections).apply(0xc0000108a0, 0xc03f594900, 0xc041176a80)
2021-03-31 07:34:57	
goroutine 318 [running]:
2021-03-31 07:34:57	

2021-03-31 07:34:57	
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x87c523]
2021-03-31 07:34:57	
panic: runtime error: invalid memory address or nil pointer dereference