Update string.go (#496)

* Update string.go

more reasonable

* test

string.go

@@ -58,7 +58,7 @@ func Substring[T ~string](str T, offset int, length uint) T {
 		}
 	}
 
-	if offset > size {
+	if offset >= size {
 		return Empty[T]()
 	}
 

string_test.go

@@ -76,6 +76,7 @@ func TestSubstring(t *testing.T) {
 	str12 := Substring("hello", -4, math.MaxUint)
 	str13 := Substring("🏠🐶🐱", 0, 2)
 	str14 := Substring("你好，世界", 0, 3)
+	str15 := Substring("hello", 5, 1)
 
 	is.Equal("", str1)
 	is.Equal("", str2)
@@ -91,6 +92,7 @@ func TestSubstring(t *testing.T) {
 	is.Equal("ello", str12)
 	is.Equal("🏠🐶", str13)
 	is.Equal("你好，", str14)
+	is.Equal("", str15)
 }
 
 func TestRuneLength(t *testing.T) {