rust/tor_util: drop unsafe block in cstr!

This is unnecessary just to get an empty string, there's Default::default().

Fix on 8fff331bb095dc6f5e2fe2ecfc9ab08ea9e2fe97.

changes/ticket28077

@@ -0,0 +1,3 @@
+  o Code simplification and refactoring:
+    - Remove unnecessarily unsafe code from the rust macro cstr!. Closes
+      ticket 28077.

src/rust/tor_util/strings.rs

@@ -105,11 +105,7 @@ macro_rules! cstr {
     ($($bytes:expr),*) => (
         ::std::ffi::CStr::from_bytes_with_nul(
             concat!($($bytes),*, "\0").as_bytes()
-        ).unwrap_or(
-            unsafe{
-                ::std::ffi::CStr::from_bytes_with_nul_unchecked(b"\0")
-            }
-        )
+        ).unwrap_or_default()
     )
 }