Initialize empty std::string values with a null terminator (#478)

The `std::string` rules model strings as null-terminated vectors, but
default initialization produced an empty vector. Operations such as
`length()` then incorrectly assume a terminator exists.

- **Type rules:** Initialize default strings with `vec![0]` in both
unsafe and refcount models.
- **Regression coverage:** Add a default-constructed string case that
checks `length()`.

```cpp
std::string empty;
assert(empty.length() == 0);
```

<!-- START COPILOT CODING AGENT SUFFIX -->

- Fixes #455

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: Nuno Lopes <nuno.lopes@tecnico.ulisboa.pt>

Author: Copilot

rules/string/tgt_refcount.rs

@@ -6,7 +6,7 @@ use std::cell::RefCell;
 use std::rc::Rc;
 
 fn t1() -> Vec<i8> {
-    Vec::new()
+    vec![0]
 }
 
 fn t2() -> Ptr<i8> {

rules/string/tgt_unsafe.rs

@@ -2,7 +2,7 @@
 // Distributed under the MIT license that can be found in the LICENSE file.
 
 fn t1() -> Vec<libc::c_char> {
-    Vec::new()
+    vec![0]
 }
 
 fn t2() -> *mut libc::c_char {

tests/unit/out/refcount/string.rs

@@ -11,6 +11,8 @@ pub fn main() {
     std::process::exit(main_0());
 }
 fn main_0() -> i32 {
+    let mut empty: Vec<i8> = vec![0];
+    assert!(((empty.len() - 1) == 0_usize));
     let s1: Value<Vec<i8>> = Rc::new(RefCell::new({
         let mut __bytes = Ptr::<i8>::from_string_literal(b"hello").to_c_bytes();
         __bytes.push(0);

tests/unit/out/unsafe/string.rs

@@ -13,6 +13,8 @@ pub fn main() {
     }
 }
 unsafe fn main_0() -> i32 {
+    let mut empty: Vec<libc::c_char> = vec![0];
+    assert!(((empty.len() - 1) == (0_usize)));
     let mut s1: Vec<libc::c_char> = {
         let s = c"hello".as_ptr();
         std::slice::from_raw_parts(s, (0..).take_while(|&i| *s.add(i) != 0).count() + 1).to_vec()

tests/unit/string.cpp

@@ -3,6 +3,8 @@
 #include <string>
 
 int main() {
+  std::string empty;
+  assert(empty.length() == 0);
   std::string s1 = "hello";
   assert(s1.length() == 5);
   assert(s1.size() == s1.length());