msxml3: Return NULL instead of empty string in IXMLElement::get_tagName.

dlls/msxml3/tests/xmldoc.c

@@ -893,7 +893,7 @@ static void test_xmlelem(void)
 
     hr = IXMLElement_get_tagName(element, &str);
     EXPECT_HR(hr, S_OK);
-    ok(lstrlenW(str) == 0, "Expected empty tag name\n");
+    ok(!str, "Expected empty tag name, got %s\n", wine_dbgstr_w(str));
     SysFreeString(str);
 
     parent = (IXMLElement *)0xdeadbeef;

dlls/msxml3/xmlelem.c

@@ -189,8 +189,12 @@ static HRESULT WINAPI xmlelem_get_tagName(IXMLElement *iface, BSTR *p)
     if (!p)
         return E_INVALIDARG;
 
-    *p = bstr_from_xmlChar(This->node->name);
-    CharUpperBuffW(*p, SysStringLen(*p));
+    if (*This->node->name) {
+        *p = bstr_from_xmlChar(This->node->name);
+        CharUpperBuffW(*p, SysStringLen(*p));
+    }else {
+        *p = NULL;
+    }
 
     TRACE("returning %s\n", debugstr_w(*p));