Return empty array for SRANDMEMBER on nonexistent key

If the count argument is given, Redis returns an empty array if the key
does not exist, unlike single call which returns nil. Mimic this
behavior more precisely.

cmd_set.go

@@ -582,6 +582,10 @@ func (m *Miniredis) cmdSrandmember(c *server.Peer, cmd string, args []string) {
 		db := m.db(ctx.selectedDB)
 
 		if !db.exists(key) {
+			if withCount {
+				c.WriteLen(0)
+				return
+			}
 			c.WriteNull()
 			return
 		}

cmd_set_test.go

@@ -381,6 +381,12 @@ func TestSrandmember(t *testing.T) {
 		"SRANDMEMBER", "nosuch",
 	)
 
+	// a nonexisting key with count
+	mustDo(t, c,
+		"SRANDMEMBER", "nosuch", "1",
+		proto.Strings(),
+	)
+
 	t.Run("errors", func(t *testing.T) {
 		s.SetAdd("chk", "aap", "noot")
 		s.Set("str", "value")