Fix std::array default initializer generating empty Vec instead of sized Vec (#337)

`std::array<int, 3> v = {};` incorrectly generated `vec![]` (0 elements)
instead of a 3-element vector with default values.

```cpp
std::array<int, 3> v = {};
```
Before:
```rust
let mut v: Vec<i32> = vec![];
```
After:
```rust
let mut v: Vec<i32> = std::array::from_fn::<_, 3, _>(|_| Default::default()).to_vec();
```

- Fixes #313

---------

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

cpp2rust/converter/converter.cpp

@@ -3102,11 +3102,11 @@ bool Converter::VisitInitListExpr(clang::InitListExpr *expr) {
   } else if (qual_type->isRecordType()) {
     const auto *record = qual_type->getAsRecordDecl();
     if (record->getQualifiedNameAsString() == "std::array") {
-      StrCat("vec!");
       if (auto init = clang::dyn_cast<clang::InitListExpr>(expr->getInit(0))) {
+        StrCat("vec!");
         VisitInitListExpr(init);
       } else {
-        StrCat("[]");
+        StrCat(GetArrayDefaultAsString(qual_type));
       }
       return false;
     }

cpp2rust/converter/models/converter_refcount.cpp

@@ -1515,12 +1515,12 @@ bool ConverterRefCount::VisitInitListExpr(clang::InitListExpr *expr) {
   if (qual_type->isRecordType()) {
     const auto *record = qual_type->getAsRecordDecl();
     if (record->getQualifiedNameAsString() == "std::array") {
-      StrCat("vec!");
       if (auto init = clang::dyn_cast<clang::InitListExpr>(expr->getInit(0))) {
+        StrCat("vec!");
         PushConversionKind push(*this, ConversionKind::Unboxed);
         ConverterRefCount::VisitInitListExpr(init);
       } else {
-        StrCat("[]");
+        StrCat(GetArrayDefaultAsString(qual_type));
       }
       computed_expr_type_ = ComputedExprType::FreshValue;
       return false;

tests/unit/out/refcount/empty_array_init.rs

@@ -10,6 +10,8 @@ pub fn main() {
     std::process::exit(main_0());
 }
 fn main_0() -> i32 {
-    let vec_: Value<Vec<i32>> = Rc::new(RefCell::new(vec![]));
+    let vec_: Value<Vec<i32>> = Rc::new(RefCell::new(
+        std::array::from_fn::<_, 3, _>(|_| Default::default()).to_vec(),
+    ));
     return 0;
 }

tests/unit/out/unsafe/empty_array_init.rs

@@ -12,6 +12,6 @@ pub fn main() {
     }
 }
 unsafe fn main_0() -> i32 {
-    let mut vec_: Vec<i32> = vec![];
+    let mut vec_: Vec<i32> = std::array::from_fn::<_, 3, _>(|_| Default::default()).to_vec();
     return 0;
 }