fix: optimization: PVS-Studio warning V815

The 'empty()' method is more efficient for checking if a string is empty compared to comparing it with an empty string literal.

The default constructor 'std::wstring()' is more efficient for creating an empty string compared to using a string literal.

The 'clear()' method is more efficient for clearing a std::wstring compared to assigning it an empty string literal.

Reported by: PVS-Studio
Signed-off-by: Vyacheslav Ivanov <islavaivanov76@gmail.com>

src/graphics/ui/elements/InventoryView.cpp

@@ -126,7 +126,7 @@ void SlotView::draw(const DrawContext* pctx, Assets* assets) {
                 langs::get(util::str2wstr_utf8(def->caption))
             );
         } else {
-            tooltip = L"";
+            tooltip.clear();
         }
     }
     prevItem = itemid;