Fix incorrect passing of a char pointer which gets converted to bool

Revealed by a warning from our Clang plug-in: "pointer 'const char *'
implicitly converted to bool [loplugin]"

I love --enable-werror and loplugin.

Change-Id: Iece0bda4325cd1e5d1333affffef12396e5dfb7d

sfx2/source/doc/docfac.cxx

@@ -371,7 +371,7 @@ OUString SfxObjectFactory::GetFactoryURL() const
 {
     OUStringBuffer aURLComposer;
     aURLComposer.append("private:factory/");
-    aURLComposer.append(GetShortName());
+    aURLComposer.appendAscii(GetShortName());
     return aURLComposer.makeStringAndClear();
 }