string: fix contains behaviour (closes #5371)

vlib/builtin/string.v

@@ -716,6 +716,9 @@ pub fn (s string) count(substr string) int {
 }
 
 pub fn (s string) contains(p string) bool {
+	if p.len == 0 {
+		return true
+	}
 	s.index(p) or {
 		return false
 	}

vlib/builtin/string_test.v

@@ -357,6 +357,8 @@ fn test_contains() {
 	s := 'view.v'
 	assert s.contains('vi')
 	assert !s.contains('random')
+	assert ''.contains('')
+	assert 'abc'.contains('')
 }
 
 fn test_arr_contains() {