fix: possible nil pointer dereference in storage queue calculation

[pboros]

Fixes possible nil pointer deference in storage queue calculation.

Example crash that this fixes:

panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x0 pc=0x856306]

goroutine 20 [running]:
github.com/tigrisdata/fdb-exporter/metrics.(*ProcessesMetricGroup).GetMetrics(0xc0002b15f0, 0xc00029e220)
	/go/src/github.com/tigrisdata/fdb-exporter/metrics/group_processes.go:242 +0x1aa6
github.com/tigrisdata/fdb-exporter/metrics.(*MetricReporter).collectOnce(0xc0002b4000)
	/go/src/github.com/tigrisdata/fdb-exporter/metrics/metric_reporter.go:114 +0x112
github.com/tigrisdata/fdb-exporter/metrics.(*MetricProvider).Collect(0xc00029c010)
	/go/src/github.com/tigrisdata/fdb-exporter/metrics/metric_provider.go:63 +0x4d
created by main.main in goroutine 1
	/go/src/github.com/tigrisdata/fdb-exporter/main.go:30 +0x119