core: list return empty array instead of null, when no cores are installed (#2691)

internal/cli/core/list.go

@@ -88,9 +88,9 @@ func GetList(ctx context.Context, srv rpc.ArduinoCoreServiceServer, inst *rpc.In
 }
 
 func newCoreListResult(in []*rpc.PlatformSummary, updatableOnly bool) *coreListResult {
-	res := &coreListResult{updatableOnly: updatableOnly}
-	for _, platformSummary := range in {
-		res.Platforms = append(res.Platforms, result.NewPlatformSummary(platformSummary))
+	res := &coreListResult{updatableOnly: updatableOnly, Platforms: make([]*result.PlatformSummary, len(in))}
+	for i, platformSummary := range in {
+		res.Platforms[i] = result.NewPlatformSummary(platformSummary)
 	}
 	return res
 }

internal/integrationtest/core/core_test.go

@@ -1174,6 +1174,7 @@ func TestCoreListWhenNoPlatformAreInstalled(t *testing.T) {
 	stdout, _, err := cli.Run("core", "list", "--json")
 	require.NoError(t, err)
 	requirejson.Query(t, stdout, `.platforms | length`, `0`)
+	requirejson.Query(t, stdout, `.platforms | select(.!=null)`, `[]`)
 
 	stdout, _, err = cli.Run("core", "list")
 	require.NoError(t, err)