fix(detected_fields): always return empty array as `null` (#14112)

pkg/querier/querier.go

@@ -1113,13 +1113,16 @@ func (q *SingleTenantQuerier) DetectedFields(ctx context.Context, req *logproto.
 			level.Warn(q.logger).Log("msg", "failed to marshal hyperloglog sketch", "err", err)
 			continue
 		}
-
+		p := v.parsers
+		if len(p) == 0 {
+			p = nil
+		}
 		fields[fieldCount] = &logproto.DetectedField{
 			Label:       k,
 			Type:        v.fieldType,
 			Cardinality: v.Estimate(),
 			Sketch:      sketch,
-			Parsers:     v.parsers,
+			Parsers:     p,
 		}
 
 		fieldCount++

pkg/querier/querier_test.go

@@ -1959,8 +1959,8 @@ func TestQuerier_DetectedFields(t *testing.T) {
 		assert.Equal(t, []string{"logfmt"}, durationField.Parsers)
 		assert.Equal(t, []string{"logfmt"}, floatField.Parsers)
 		assert.Equal(t, []string{"logfmt"}, evenField.Parsers)
-		assert.Equal(t, []string{}, constantField.Parsers)
-		assert.Equal(t, []string{}, variableField.Parsers)
+		assert.Equal(t, []string(nil), constantField.Parsers)
+		assert.Equal(t, []string(nil), variableField.Parsers)
 	},
 	)
 

pkg/storage/detected/fields.go

@@ -44,6 +44,9 @@ func (f *UnmarshaledDetectedField) Merge(df *logproto.DetectedField) error {
 	f.Parsers = append(f.Parsers, df.Parsers...)
 	slices.Sort(f.Parsers)
 	f.Parsers = slices.Compact(f.Parsers)
+	if len(f.Parsers) == 0 {
+		f.Parsers = nil
+	}
 
 	return f.Sketch.Merge(sketch)
 }