string: fix capitalize

vlib/builtin/string.v

@@ -752,6 +752,9 @@ pub fn (s string) to_upper() string {
 }
 
 pub fn (s string) capitalize() string {
+	if s.len == 0 {
+		return ''
+	}
 	sl := s.to_lower()
 	cap := sl[0].str().to_upper() + sl.right(1)
 	return cap

vlib/builtin/string_test.v

@@ -503,6 +503,8 @@ fn test_capitalize() {
 	assert s.capitalize() == 'Test'
     s = 'i am ray'
 	assert s.capitalize() == 'I am ray'
+	s = ''
+	assert s.capitalize() == ''
 }
 
 fn test_title() {