Don't return NULL from {Boxed}PrimitiveTypeName.

The behavior of the string ctor is undefined when you pass NULL. This
is checked strictly in C++11, so fails to compile.

Change-Id: Id5e0984ad1d37f2d504f7c42ac23e52ed4a58903

src/google/protobuf/compiler/javanano/javanano_helpers.cc

@@ -336,14 +336,14 @@ string PrimitiveTypeName(JavaType type) {
     case JAVATYPE_STRING : return "java.lang.String";
     case JAVATYPE_BYTES  : return "byte[]";
     case JAVATYPE_ENUM   : return "int";
-    case JAVATYPE_MESSAGE: return NULL;
+    case JAVATYPE_MESSAGE: return "";
 
     // No default because we want the compiler to complain if any new
     // JavaTypes are added.
   }
 
   GOOGLE_LOG(FATAL) << "Can't get here.";
-  return NULL;
+  return "";
 }
 
 string BoxedPrimitiveTypeName(JavaType type) {
@@ -356,14 +356,14 @@ string BoxedPrimitiveTypeName(JavaType type) {
     case JAVATYPE_STRING : return "java.lang.String";
     case JAVATYPE_BYTES  : return "byte[]";
     case JAVATYPE_ENUM   : return "java.lang.Integer";
-    case JAVATYPE_MESSAGE: return NULL;
+    case JAVATYPE_MESSAGE: return "";
 
     // No default because we want the compiler to complain if any new
     // JavaTypes are added.
   }
 
   GOOGLE_LOG(FATAL) << "Can't get here.";
-  return NULL;
+  return "";
 }
 
 string EmptyArrayName(const Params& params, const FieldDescriptor* field) {