Heartbeat#sendEvent(m): nil-check m before dereferencing it

[Al2Klimov]

as it can be nil.

[julianbrost]

For reference, this is what the PR is supposed to fix:

2023-01-19T14:52:34.040Z        INFO    runtime-updates Upserted 1 HostState items
2023-01-19T14:53:27.647Z        WARN    heartbeat       Lost Icinga heartbeat   {"timeout": "1m0s"}
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x2f39a4]

goroutine 13 [running]:
github.com/icinga/icingadb/pkg/icingaredis.(*Heartbeat).sendEvent(0x4000156410, 0x0)
        /go/src/github.com/Icinga/icingadb/pkg/icingaredis/heartbeat.go:184 +0xa4
github.com/icinga/icingadb/pkg/icingaredis.(*Heartbeat).controller.func2()
        /go/src/github.com/Icinga/icingadb/pkg/icingaredis/heartbeat.go:147 +0x170
golang.org/x/sync/errgroup.(*Group).Go.func1()
        /go/pkg/mod/golang.org/x/sync@v0.0.0-20210220032951-036812b2e83c/errgroup/errgroup.go:57 +0x60
created by golang.org/x/sync/errgroup.(*Group).Go
        /go/pkg/mod/golang.org/x/sync@v0.0.0-20210220032951-036812b2e83c/errgroup/errgroup.go:54 +0x8c
exit status 2