[nspcc-dev#633] object: Disallow empty attribute values

Values of object attributes must not be empty according to NeoFS
specification.

Make `FormatValidator.Validate` method to return an error if at least one
attribute has empty value.

Signed-off-by: Leonard Lyubich <leonard@nspcc.ru>

pkg/core/object/fmt.go

@@ -231,7 +231,10 @@ func expirationEpochAttribute(obj *Object) (uint64, error) {
 	return 0, errNoExpirationEpoch
 }
 
-var errDuplAttr = errors.New("duplication of attributes detected")
+var (
+	errDuplAttr     = errors.New("duplication of attributes detected")
+	errEmptyAttrVal = errors.New("empty attribute value")
+)
 
 func (v *FormatValidator) checkAttributes(obj *Object) error {
 	as := obj.Attributes()
@@ -245,6 +248,11 @@ func (v *FormatValidator) checkAttributes(obj *Object) error {
 			return errDuplAttr
 		}
 
+		val := a.Value()
+		if val == "" {
+			return errEmptyAttrVal
+		}
+
 		mUnique[key] = struct{}{}
 	}
 

pkg/core/object/fmt_test.go

@@ -242,5 +242,17 @@ func TestFormatValidator_Validate(t *testing.T) {
 			err = v.checkAttributes(obj.Object())
 			require.Equal(t, errDuplAttr, err)
 		})
+
+		t.Run("empty value", func(t *testing.T) {
+			obj := blankValidObject(t, ownerKey)
+
+			a := object.NewAttribute()
+			a.SetKey("key")
+
+			obj.SetAttributes(a)
+
+			err := v.checkAttributes(obj.Object())
+			require.Equal(t, errEmptyAttrVal, err)
+		})
 	})
 }