xml: fix build

empty is already not a nil.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/5376098

src/pkg/encoding/xml/xml_test.go

@@ -42,17 +42,17 @@ var rawTokens = []Token{
 	CharData([]byte("World <>'\" 白鵬翔")),
 	EndElement{Name{"", "hello"}},
 	CharData([]byte("\n  ")),
-	StartElement{Name{"", "goodbye"}, nil},
+	StartElement{Name{"", "goodbye"}, []Attr{}},
 	EndElement{Name{"", "goodbye"}},
 	CharData([]byte("\n  ")),
 	StartElement{Name{"", "outer"}, []Attr{{Name{"foo", "attr"}, "value"}, {Name{"xmlns", "tag"}, "ns4"}}},
 	CharData([]byte("\n    ")),
-	StartElement{Name{"", "inner"}, nil},
+	StartElement{Name{"", "inner"}, []Attr{}},
 	EndElement{Name{"", "inner"}},
 	CharData([]byte("\n  ")),
 	EndElement{Name{"", "outer"}},
 	CharData([]byte("\n  ")),
-	StartElement{Name{"tag", "name"}, nil},
+	StartElement{Name{"tag", "name"}, []Attr{}},
 	CharData([]byte("\n    ")),
 	CharData([]byte("Some text here.")),
 	CharData([]byte("\n  ")),
@@ -76,17 +76,17 @@ var cookedTokens = []Token{
 	CharData([]byte("World <>'\" 白鵬翔")),
 	EndElement{Name{"ns2", "hello"}},
 	CharData([]byte("\n  ")),
-	StartElement{Name{"ns2", "goodbye"}, nil},
+	StartElement{Name{"ns2", "goodbye"}, []Attr{}},
 	EndElement{Name{"ns2", "goodbye"}},
 	CharData([]byte("\n  ")),
 	StartElement{Name{"ns2", "outer"}, []Attr{{Name{"ns1", "attr"}, "value"}, {Name{"xmlns", "tag"}, "ns4"}}},
 	CharData([]byte("\n    ")),
-	StartElement{Name{"ns2", "inner"}, nil},
+	StartElement{Name{"ns2", "inner"}, []Attr{}},
 	EndElement{Name{"ns2", "inner"}},
 	CharData([]byte("\n  ")),
 	EndElement{Name{"ns2", "outer"}},
 	CharData([]byte("\n  ")),
-	StartElement{Name{"ns3", "name"}, nil},
+	StartElement{Name{"ns3", "name"}, []Attr{}},
 	CharData([]byte("\n    ")),
 	CharData([]byte("Some text here.")),
 	CharData([]byte("\n  ")),
@@ -104,7 +104,7 @@ var rawTokensAltEncoding = []Token{
 	CharData([]byte("\n")),
 	ProcInst{"xml", []byte(`version="1.0" encoding="x-testing-uppercase"`)},
 	CharData([]byte("\n")),
-	StartElement{Name{"", "tag"}, nil},
+	StartElement{Name{"", "tag"}, []Attr{}},
 	CharData([]byte("value")),
 	EndElement{Name{"", "tag"}},
 }