fix!: handle sql/driver.Valuer types properly in slogjson (#219)

Currently, if a field like `sql.NullInt32` has `Valid: False`, `sloghuman` will export it's value as `<nil>`, regardless of it's `String`.
This is because it checks `(driver.Valuer).Value()`.

However, `slogjson` currently sets the value to the json string of the raw struct:
```json
{
  "fields": {
      "Code": "{Int32:0 Valid:false}",
      "ValidCode": "{Int32:12 Valid:true}"
  }
}
```

This PR handles this case by first checking if the type implements `sql/driver.Valuer`. If `Valid` is `false` then a JSON `null` value is produced:
```json
{
  "fields": {
      "Code": null,
      "ValidCode": 12
  }
}
```
This matches the behaviour of `sloghuman`.

This is technically a breaking change, as these types are now `T | null` instead of `String`, where `T` is the corresponding JSON type of `sql.Null<V>`

Author: ethanndickson

map.go

@@ -2,6 +2,7 @@ package slog
 
 import (
 	"bytes"
+	"database/sql/driver"
 	"encoding/json"
 	"fmt"
 	"reflect"
@@ -70,6 +71,14 @@ func marshalList(rv reflect.Value) []byte {
 }
 
 func encode(v interface{}) []byte {
+	if vr, ok := v.(driver.Valuer); ok {
+		var err error
+		v, err = vr.Value()
+		if err != nil {
+			return encodeJSON(fmt.Sprintf("error calling Value: %v", err))
+		}
+	}
+
 	switch v := v.(type) {
 	case json.Marshaler:
 		return encodeJSON(v)

map_test.go

@@ -62,12 +62,12 @@ func TestMap(t *testing.T) {
 				{
 					"msg": "wrap1",
 					"fun": "cdr.dev/slog_test.TestMap.func2",
-					"loc": "`+mapTestFile+`:41" 
+					"loc": "`+mapTestFile+`:41"
 				},
 				{
 					"msg": "wrap2",
 					"fun": "cdr.dev/slog_test.TestMap.func2",
-					"loc": "`+mapTestFile+`:42" 
+					"loc": "`+mapTestFile+`:42"
 				},
 				"EOF"
 			],
@@ -93,7 +93,7 @@ func TestMap(t *testing.T) {
 					{
 						"msg": "failed to marshal to JSON",
 						"fun": "cdr.dev/slog.encodeJSON",
-						"loc": "`+mapTestFile+`:131"
+						"loc": "`+mapTestFile+`:140"
 					},
 					"json: error calling MarshalJSON for type slog_test.complexJSON: json: unsupported type: complex128"
 				],

sloggers/slogjson/slogjson_test.go

@@ -3,6 +3,7 @@ package slogjson_test
 import (
 	"bytes"
 	"context"
+	"database/sql"
 	"fmt"
 	"runtime"
 	"testing"
@@ -33,7 +34,33 @@ func TestMake(t *testing.T) {
 	l.Error(ctx, "line1\n\nline2", slog.F("wowow", "me\nyou"))
 
 	j := entryjson.Filter(b.String(), "ts")
-	exp := fmt.Sprintf(`{"level":"ERROR","msg":"line1\n\nline2","caller":"%v:33","func":"cdr.dev/slog/sloggers/slogjson_test.TestMake","logger_names":["named"],"trace":"%v","span":"%v","fields":{"wowow":"me\nyou"}}
+	exp := fmt.Sprintf(`{"level":"ERROR","msg":"line1\n\nline2","caller":"%v:34","func":"cdr.dev/slog/sloggers/slogjson_test.TestMake","logger_names":["named"],"trace":"%v","span":"%v","fields":{"wowow":"me\nyou"}}
 `, slogjsonTestFile, span.SpanContext().TraceID().String(), span.SpanContext().SpanID().String())
 	assert.Equal(t, "entry", exp, j)
 }
+
+func TestNoDriverValue(t *testing.T) {
+	t.Parallel()
+
+	b := &bytes.Buffer{}
+	l := slog.Make(slogjson.Sink(b))
+	l = l.Named("named")
+	validField := sql.NullString{
+		String: "cat",
+		Valid:  true,
+	}
+	invalidField := sql.NullString{
+		String: "dog",
+		Valid:  false,
+	}
+	validInt := sql.NullInt64{
+		Int64: 42,
+		Valid: true,
+	}
+	l.Error(bg, "error!", slog.F("inval", invalidField), slog.F("val", validField), slog.F("int", validInt))
+
+	j := entryjson.Filter(b.String(), "ts")
+	exp := fmt.Sprintf(`{"level":"ERROR","msg":"error!","caller":"%v:60","func":"cdr.dev/slog/sloggers/slogjson_test.TestNoDriverValue","logger_names":["named"],"fields":{"inval":null,"val":"cat","int":42}}
+`, slogjsonTestFile)
+	assert.Equal(t, "entry", exp, j)
+}