propsys: Add VT_EMPTY support for PropVariantToStringAlloc().

dlls/propsys/propvar.c

@@ -314,6 +314,7 @@ HRESULT WINAPI PropVariantToStringAlloc(REFPROPVARIANT propvarIn, WCHAR **ret)
 
     switch(propvarIn->vt)
     {
+        case VT_EMPTY:
         case VT_NULL:
             res = CoTaskMemAlloc(1*sizeof(WCHAR));
             res[0] = '\0';

dlls/propsys/tests/propsys.c

@@ -641,6 +641,12 @@ static void test_PropVariantToStringAlloc(void)
     ok(!lstrcmpW(str, topicW), "got %s\n", wine_dbgstr_w(str));
     CoTaskMemFree(str);
     PropVariantClear(&prop);
+
+    prop.vt = VT_EMPTY;
+    hres = PropVariantToStringAlloc(&prop, &str);
+    ok(hres == S_OK, "returned %x\n", hres);
+    ok(!lstrcmpW(str, emptyW), "got %s\n", wine_dbgstr_w(str));
+    CoTaskMemFree(str);
 }
 
 static void test_PropVariantCompare(void)