expvar: document that Get returns nil for non-existent vars

Also added a test to ensure the behavior.

Fixes #14150

Change-Id: Ib3ee9fdae59826fa594ce1be3c49b51d740b56eb
Reviewed-on: https://go-review.googlesource.com/19915Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/expvar/expvar.go

@@ -258,7 +258,8 @@ func Publish(name string, v Var) {
 	sort.Strings(varKeys)
 }
 
-// Get retrieves a named exported variable.
+// Get retrieves a named exported variable. It returns nil if the name has
+// not been registered.
 func Get(name string) Var {
 	mutex.RLock()
 	defer mutex.RUnlock()

src/expvar/expvar_test.go

@@ -26,6 +26,14 @@ func RemoveAll() {
 	varKeys = nil
 }
 
+func TestNil(t *testing.T) {
+	RemoveAll()
+	val := Get("missing")
+	if val != nil {
+		t.Errorf("got %v, want nil", val)
+	}
+}
+
 func TestInt(t *testing.T) {
 	RemoveAll()
 	reqs := NewInt("requests")