pkg/rules/rulespb: fix JSON marshaling for nil slices

[simonpasquier]

Go's encoding/json turns nil slices into null instead of []. This
is problematic for consumers expecting that the alerts field is a JSON
array. The same is true for empty rule groups.

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user (the feature hasn't been released yet).

Changes

The solution is to enforce that nil slices are converted to zero-length
slices when encoding.

Verification

I've updated the unit tests to cover the issue.

[bwplotka]

Very valid indeed. I think we were trying to fix this by returning make([]string) from various algorithms instead of nil which is kind of wrong (unnecessary allocs). I don't remember if it was actually merged at the end but I would see approach like yours to see more in future as a way to go 🤗
Thanks!

[bwplotka]

Tests are failing for a good reason I think (:

[simonpasquier]

@bwplotka I've fixed tests in pkg/query/api/v1_test.go indeed. The e2e failure is more cryptic for me...

2020-07-13T09:51:15.5978723Z ##[error]    TestQueryExternalPrefixWithoutReverseProxy: query_test.go:278: �[31mquery_test.go:278:
2020-07-13T09:51:15.5979200Z         
2020-07-13T09:51:15.5980293Z          unexpected error: some network requests failed: net::ERR_NETWORK_CHANGED; net::ERR_NETWORK_CHANGED; net::ERR_NETWORK_CHANGED; net::ERR_NETWORK_CHANGED; net::ERR_NETWORK_CHANGED; net::ERR_NETWORK_CHANGED; net::ERR_NETWORK_CHANGED�[39m
2020-07-13T09:51:15.5980510Z         
2020-07-13T09:51:15.5980809Z Killing querier-1
2020-07-13T09:51:15.5980982Z Error: No such network: e2e_test_query
2020-07-13T09:51:15.5981064Z 
2020-07-13T09:51:15.5981286Z Unable to remove docker network e2e_test_query : exit status 1

[bwplotka]

Looks like flak, retried

[bwplotka]

Thanks!