Make LuaString.ToString() return null for null pointer

Author: Quahu

src/Laylua/Library/Entities/Reference/LuaReference.cs

@@ -200,7 +200,7 @@ public override string ToString()
         {
             PushValue(this);
             var L = Lua.GetStatePointer();
-            return luaL_tostring(L, -1).ToString();
+            return luaL_tostring(L, -1).ToString() ?? "<invalid>";
         }
     }
 

src/Laylua/Library/LuaException.cs

@@ -72,7 +72,7 @@ internal static unsafe bool TryGetError(lua_State* L, out (string? Message, Exce
 
         if (type == LuaType.String)
         {
-            error = (lua_tostring(L, -1).ToString().Replace("\r", "").Replace("\n", ""), Exception: null);
+            error = (lua_tostring(L, -1).ToString()?.Replace("\r", "").Replace("\n", ""), Exception: null);
             return true;
         }
 

src/Laylua/Library/Marshaler/DefaultLuaMarshaler.TryGetValue.cs

@@ -234,7 +234,7 @@ public override bool TryGetValue<T>(LuaThread lua, int stackIndex, out T? obj)
                 var nativeStringValue = lua_tostring(L, stackIndex);
                 if (clrType == typeof(string) || clrType == typeof(object))
                 {
-                    obj = (T) (object) nativeStringValue.ToString();
+                    obj = (T?) (object?) nativeStringValue.ToString();
                     return true;
                 }
 

src/Laylua/Moon/Native/LuaString.cs

@@ -81,11 +81,11 @@ public int GetChars(Span<char> destination)
     ///     Returns a <see cref="string"/> created from this <see cref="LuaString"/>.
     /// </summary>
     /// <returns> The created <see cref="string"/>. </returns>
-    public override string ToString()
+    public override string? ToString()
     {
         return Pointer != null
             ? Encoding.UTF8.GetString(Pointer, (int) Length)
-            : "<invalid string pointer>";
+            : null;
     }
 
     /// <inheritdoc/>