WaE: rewrite call of rtl::OUString::operator= ...

... with empty string constant argument as call of rtl::OUString::clear
[loplugin:stringconstant].

Actually, no need to clear the string here as it already is empty, surely?

Change-Id: Ib4b1a6e1b0c0f59c1c964fe58d26058b492c3821

basic/source/runtime/methods.cxx

@@ -1281,7 +1281,7 @@ RTLFUNC(Mid)
                 OUString aResultStr;
                 if (nStartPos > aArgStr.getLength())
                 {
-                    aResultStr = "";
+                    // do nothing
                 }
                 else if(nArgCount == 2)
                 {