Check that metric_relabel_configs has non-nil configuration in runtime config

[pstibrany]

What this PR does

This PR fixes panic in distributor which happens when user writes invalid metric_relabel_configs into overrides like this:

overrides:
  anonymous:
    metric_relabel_configs:
      -

This was accepted as valid runtime config file, but metric relabel configs translated to []*relabel.Config{nil}. When distributor uses this, it panics (line numbers match main at ef085cd):

2023/01/06 09:42:40 http: panic serving 172.18.0.5:42300: runtime error: invalid memory address or nil pointer dereference
goroutine 15715 [running]:
net/http.(*conn).serve.func1()
	/opt/homebrew/Cellar/go/1.19.2/libexec/src/net/http/server.go:1850 +0x120
panic({0x1fbfd80, 0x37a5770})
	/opt/homebrew/Cellar/go/1.19.2/libexec/src/runtime/panic.go:890 +0x264
github.com/opentracing-contrib/go-stdlib/nethttp.MiddlewareFunc.func5.1()
	/Users/peter/Grafana/mimir/vendor/github.com/opentracing-contrib/go-stdlib/nethttp/server.go:150 +0x1a0
panic({0x1fbfd80, 0x37a5770})
	/opt/homebrew/Cellar/go/1.19.2/libexec/src/runtime/panic.go:890 +0x264
github.com/prometheus/prometheus/model/relabel.relabel({0x40019301c0, 0xb, 0xe}, 0x0, 0x400082f540)
	/Users/peter/Grafana/mimir/vendor/github.com/prometheus/prometheus/model/relabel/relabel.go:222 +0x7c
github.com/prometheus/prometheus/model/relabel.Process({0x40019301c0, 0xb, 0xe}, {0x4001fb40d8, 0x1, 0x1})
	/Users/peter/Grafana/mimir/vendor/github.com/prometheus/prometheus/model/relabel/relabel.go:211 +0xb0
github.com/grafana/mimir/pkg/distributor.(*Distributor).prePushRelabelMiddleware.func1({0x2945388, 0x40005c5ad0}, 0x4000f97680)
	/Users/peter/Grafana/mimir/pkg/distributor/distributor.go:801 +0x2ac
...

Which issue(s) this PR fixes or relates to

No issue created.

Checklist

• Tests updated
• [na] Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]