### Describe Your Changes
Currently it the metricID list is empty it won't be mashalled and as the
result won't be put into the tagFiltersToMetricIDsCache which causes the
cache misses for the corresponding tagFilters. In some setups this
causes severe search speed detradation (see #7009).
The empty metric IDs was covered before but then was accidentally
removed in 6c21439.
This PR restores the coverage of this case.
A new unit test can be used as a proof that empty metricID lists are not
added to the cache (just remove the fix in index_db.go and run the test
to see the result)
Also a benchmark has been added to see the implications of the
compression.
```
user@laptop:~/p/github.com/rtm0/VictoriaMetrics/01/src$ go test ./lib/storage/ -run=NONE -bench BenchmarkMarshalUnmarshalMetricIDs --loggerLevel=ERROR
goos: linux
goarch: amd64
pkg: github.com/VictoriaMetrics/VictoriaMetrics/lib/storage
cpu: 13th Gen Intel(R) Core(TM) i7-1355U
BenchmarkMarshalUnmarshalMetricIDs/numMetricIDs-0-12 3237240 363.5 ns/op 0 compression-rate
BenchmarkMarshalUnmarshalMetricIDs/numMetricIDs-1-12 2831049 451.8 ns/op 0.4706 compression-rate
BenchmarkMarshalUnmarshalMetricIDs/numMetricIDs-10-12 1152764 1009 ns/op 1.667 compression-rate
BenchmarkMarshalUnmarshalMetricIDs/numMetricIDs-100-12 297055 3998 ns/op 5.755 compression-rate
BenchmarkMarshalUnmarshalMetricIDs/numMetricIDs-1000-12 31172 34566 ns/op 8.484 compression-rate
BenchmarkMarshalUnmarshalMetricIDs/numMetricIDs-10000-12 4900 289659 ns/op 9.416 compression-rate
BenchmarkMarshalUnmarshalMetricIDs/numMetricIDs-100000-12 447 2341173 ns/op 9.456 compression-rate
BenchmarkMarshalUnmarshalMetricIDs/numMetricIDs-1000000-12 42 24926928 ns/op 9.468 compression-rate
BenchmarkMarshalUnmarshalMetricIDs/numMetricIDs-10000000-12 5 204098872 ns/op 9.467 compression-rate
PASS
ok github.com/VictoriaMetrics/VictoriaMetrics/lib/storage 15.018s
```
### Checklist
The following checks are **mandatory**:
- [x] My change adheres [VictoriaMetrics contributing
guidelines](https://docs.victoriametrics.com/contributing/).
---------
Signed-off-by: Artem Fetishev <wwctrsrx@gmail.com>
Co-authored-by: Aliaksandr Valialkin <valyala@victoriametrics.com>
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