Add nullable support throughout the whole library.

This requires reference types that can be null to be annotated by a ? operator, similar to value types.

This gives the advantage that the compiler can warn against any null dereference exceptions, of which this commits elimits a few.

To make the underlying protocol implementation not any more complicated and to eliminate existing problems, and not that precise error reporting, I replaced some return null statements with explicit Exceptions. This lead to the assumption that those core protocoll functions always return non-null objects if they do not throw, making the PLC code simpler.

Adjust some NotConnected tests to look for explicit PlcException instead of NullReferenceException.

S7.Net/PLCHelpers.cs

@@ -82,7 +82,7 @@ namespace S7.Net
         /// <param name="varCount"></param>
         /// <param name="bitAdr"></param>
         /// <returns></returns>
-        private object ParseBytes(VarType varType, byte[] bytes, int varCount, byte bitAdr = 0)
+        private object? ParseBytes(VarType varType, byte[] bytes, int varCount, byte bitAdr = 0)
         {
             if (bytes == null || bytes.Length == 0)
                 return null;