Fix unmarshaler handling of empty strings.

decode.go

diff --git a/decode.go b/decode.go
index 085cddc..b13ab9f 100644
--- a/decode.go
+++ b/decode.go
@@ -251,7 +251,7 @@
 //
 // If n holds a null value, prepare returns before doing anything.
 func (d *decoder) prepare(n *node, out reflect.Value) (newout reflect.Value, unmarshaled, good bool) {
-	if n.tag == yaml_NULL_TAG || n.kind == scalarNode && n.tag == "" && (n.value == "null" || n.value == "") {
+	if n.tag == yaml_NULL_TAG || n.kind == scalarNode && n.tag == "" && (n.value == "null" || n.value == "" && n.implicit) {
 		return out, false, false
 	}
 	again := true

decode_test.go

diff --git a/decode_test.go b/decode_test.go
index c159760..3da6fad 100644
--- a/decode_test.go
+++ b/decode_test.go
@@ -660,6 +660,7 @@
 	{`_: BAR!`, "!!str", "BAR!"},
 	{`_: "BAR!"`, "!!str", "BAR!"},
 	{"_: !!foo 'BAR!'", "!!foo", "BAR!"},
+	{`_: ""`, "!!str", ""},
 }
 
 var unmarshalerResult = map[int]error{}