Fix warnings from nullabilty

PortableExecutable/DebugTable.cs

@@ -33,6 +33,10 @@
         /// file and is not mapped into the run-time address space). If it is mapped,
         /// the RVA is its address.
         /// </summary>
+#if NET48
         public DebugDirectoryEntry[] DebugDirectoryTable;
+#else
+        public DebugDirectoryEntry[]? DebugDirectoryTable;
+#endif
     }
 }