src: do not pass nullptr to std::string ctor

fixes -Wnonnull warning on some compilers

PR-URL: #57354
Reviewed-By: Daeyeon Jeong <daeyeon.dev@gmail.com>
Reviewed-By: Yagiz Nizipli <yagiz@nizipli.com>
Reviewed-By: Chengzhong Wu <legendecas@gmail.com>
Reviewed-By: Shelley Vohr <shelley.vohr@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

Author: ckerr

src/debug_utils.cc

@@ -254,7 +254,7 @@ class Win32SymbolDebuggingContext final : public NativeSymbolDebuggingContext {
       USE(GetLastError());
 #endif  // DEBUG
     }
-    return nullptr;
+    return {};
   }
 
   SymbolInfo LookupSymbol(void* address) override {