fix nil pointer

[hzhao-wish]

Log shows that metricsReplApplyOpsTotal.Set(applyStats.Ops) throws nil pointer error.

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x8 pc=0x7a99cb]

goroutine 46 [running]:
github.com/dcu/mongodb_exporter/collector.(*ApplyStats).Export(0x0, 0xc000180420?)
	/go/src/github.com/dcu/mongodb_exporter/collector/metrics.go:315 +0x2b
github.com/dcu/mongodb_exporter/collector.(*ReplStats).Export(0xc000132320, 0x881711?)
	/go/src/github.com/dcu/mongodb_exporter/collector/metrics.go:363 +0x29
github.com/dcu/mongodb_exporter/collector.(*MetricsStats).Export(0xc0002a0140, 0xc0000fec40?)
	/go/src/github.com/dcu/mongodb_exporter/collector/metrics.go:479 +0xd9
github.com/dcu/mongodb_exporter/collector.(*ServerStatus).Export(0xc0000a3110, 0x1?)
	/go/src/github.com/dcu/mongodb_exporter/collector/server_status.go:125 +0x285
github.com/dcu/mongodb_exporter/collector.(*MongodbCollector).collectServerStatus(0xc0001af680?, 0x0?, 0x0?)
	/go/src/github.com/dcu/mongodb_exporter/collector/mongodb_collector.go:173 +0x85
github.com/dcu/mongodb_exporter/collector.(*MongodbCollector).Collect(0xc0001a4090, 0x0?)
	/go/src/github.com/dcu/mongodb_exporter/collector/mongodb_collector.go:107 +0x348
github.com/prometheus/client_golang/prometheus.(*Registry).Gather.func2({0x9370b8?, 0xc0001a4090?})
	/go/src/github.com/dcu/mongodb_exporter/vendor/github.com/prometheus/client_golang/prometheus/registry.go:433 +0x6d
created by github.com/prometheus/client_golang/prometheus.(*Registry).Gather
	/go/src/github.com/dcu/mongodb_exporter/vendor/github.com/prometheus/client_golang/prometheus/registry.go:431 +0x2d0

Check nil before calling set function. Hard to set up a mongoproxy in dev (cannot configure vault, git-sync, ...). Plan to test in toolsstage cluster.