Simply typed nil and driver.Valuer handling

* Convert typed nils to untyped nils at beginning of encoding process.
* Restore v4 json/jsonb null behavior
* Add anynil internal package

internal/anynil/anynil.go

@@ -0,0 +1,36 @@
+package anynil
+
+import "reflect"
+
+// Is returns true if value is any type of nil. e.g. nil or []byte(nil).
+func Is(value interface{}) bool {
+	if value == nil {
+		return true
+	}
+
+	refVal := reflect.ValueOf(value)
+	switch refVal.Kind() {
+	case reflect.Chan, reflect.Func, reflect.Map, reflect.Ptr, reflect.UnsafePointer, reflect.Interface, reflect.Slice:
+		return refVal.IsNil()
+	default:
+		return false
+	}
+}
+
+// Normalize converts typed nils (e.g. []byte(nil)) into untyped nil. Other values are returned unmodified.
+func Normalize(v interface{}) interface{} {
+	if Is(v) {
+		return nil
+	}
+	return v
+}
+
+// NormalizeSlice converts all typed nils (e.g. []byte(nil)) in s into untyped nils. Other values are unmodified. s is
+// mutated in place.
+func NormalizeSlice(s []interface{}) {
+	for i := range s {
+		if Is(s[i]) {
+			s[i] = nil
+		}
+	}
+}