JSON "" shouldn't unmarshal to a null null.String.

That's what the zero package is for.
This was a "feature", but inconsistent with the rest of the null package.

README.md

@@ -1,5 +1,5 @@
 ## null [![GoDoc](https://godoc.org/github.com/guregu/null?status.svg)](https://godoc.org/github.com/guregu/null) [![Coverage](http://gocover.io/_badge/github.com/guregu/null)](http://gocover.io/github.com/guregu/null)
-`import "gopkg.in/guregu/null.v2"`
+`import "gopkg.in/guregu/null.v3"`
 
 null is a library with reasonable options for dealing with nullable SQL and JSON values
 
@@ -13,7 +13,7 @@ All types implement `sql.Scanner` and `driver.Valuer`, so you can use this libra
 
 ### null package
 
-`import "gopkg.in/guregu/null.v2"`
+`import "gopkg.in/guregu/null.v3"`
 
 #### null.String
 Nullable string.
@@ -41,7 +41,7 @@ Marshals to JSON null if SQL source data is null. Uses `time.Time`'s marshaler.
 
 ### zero package
 
-`import "gopkg.in/guregu/null.v2/zero"`
+`import "gopkg.in/guregu/null.v3/zero"`
 
 #### zero.String
 Nullable int64. 

string.go

@@ -41,7 +41,7 @@ func NewString(s string, valid bool) String {
 }
 
 // UnmarshalJSON implements json.Unmarshaler.
-// It supports string and null input. Blank string input produces a null String.
+// It supports string and null input. Blank string input does not produce a null String.
 // It also supports unmarshalling a sql.NullString.
 func (s *String) UnmarshalJSON(data []byte) error {
 	var err error
@@ -60,7 +60,7 @@ func (s *String) UnmarshalJSON(data []byte) error {
 	default:
 		err = fmt.Errorf("json: cannot unmarshal %v into Go value of type null.String", reflect.TypeOf(v).Name())
 	}
-	s.Valid = (err == nil) && (s.String != "")
+	s.Valid = err == nil
 	return err
 }
 

string_test.go

@@ -52,7 +52,9 @@ func TestUnmarshalString(t *testing.T) {
 	var blank String
 	err = json.Unmarshal(blankStringJSON, &blank)
 	maybePanic(err)
-	assertNullStr(t, blank, "blank string json")
+	if !blank.Valid {
+		t.Error("blank string should be valid")
+	}
 
 	var null String
 	err = json.Unmarshal(nullJSON, &null)