Make check in XML output function a precondition

Previously this check would just lead to a silent return when
trying to output an empty XML document. As per the comment this is
actually an error.

Also fixes one instance where this function was used incorrectly in
goto-analyzer.

Author: hannes-steffenhagen-diffblue

src/goto-analyzer/static_verifier.cpp

@@ -66,7 +66,7 @@ static void static_verifier_xml(
 {
   m.status() << "Writing XML report" << messaget::eom;
 
-  xmlt xml_result;
+  xmlt xml_result{"cprover"};
 
   for(const auto &result : results)
   {

src/util/xml.cpp

@@ -34,8 +34,7 @@ void xmlt::output(std::ostream &out, unsigned indent) const
   // 'name' needs to be set, or we produce mal-formed
   // XML.
 
-  if(name.empty())
-    return;
+  PRECONDITION(!name.empty());
 
   do_indent(out, indent);