Don't load nil types when lazy loading

This can happen when processing the clone method which sets its return
type to be a void*

Author: thk123

jbmc/src/java_bytecode/ci_lazy_methods_needed.cpp

@@ -138,7 +138,9 @@ void ci_lazy_methods_neededt::gather_field_types(
     {
       const typet &element_type =
         java_array_element_type(to_struct_tag_type(class_type));
-      if(element_type.id() == ID_pointer)
+      if(
+        element_type.id() == ID_pointer &&
+        element_type.subtype().id() != ID_empty)
       {
         // This is a reference array -- mark its element type available.
         add_all_needed_classes(to_pointer_type(element_type));

jbmc/src/java_bytecode/java_types.cpp

@@ -209,7 +209,7 @@ typet java_type_from_char(char t)
   case 'f': return java_float_type();
   case 'd': return java_double_type();
   case 'z': return java_boolean_type();
-  case 'a': return java_lang_object_type();
+  case 'a': return java_reference_type(void_typet());
   default:
     UNREACHABLE;
   }