make bytes in InnerHeaderField non-nullable.

lib/src/kdbx_binary.dart

@@ -20,9 +20,9 @@ class KdbxBinary {
   int? _valueHashCode;
 
   static KdbxBinary readBinaryInnerHeader(InnerHeaderField field) {
-    final flags = field.bytes![0];
+    final flags = field.bytes[0];
     final isProtected = flags & 0x01 == 0x01;
-    final value = Uint8List.sublistView(field.bytes!, 1);
+    final value = Uint8List.sublistView(field.bytes, 1);
     return KdbxBinary(
       isInline: false,
       isProtected: isProtected,

lib/src/kdbx_header.dart

@@ -145,7 +145,7 @@ class InnerHeaderField implements HeaderFieldBase<InnerHeaderFields> {
 
   @override
   final InnerHeaderFields field;
-  final Uint8List? bytes;
+  final Uint8List bytes;
 
   String get name => field.toString();
 }
@@ -323,10 +323,10 @@ class KdbxHeader {
   void _writeInnerField(WriterHelper writer, InnerHeaderField value) {
     final field = value.field;
     _logger.finer(
-        'Writing header $field (${field.index}) (${value.bytes!.lengthInBytes})');
+        'Writing header $field (${field.index}) (${value.bytes.lengthInBytes})');
     writer.writeUint8(field.index);
-    _writeFieldSize(writer, value.bytes!.lengthInBytes);
-    writer.writeBytes(value.bytes!);
+    _writeFieldSize(writer, value.bytes.lengthInBytes);
+    writer.writeBytes(value.bytes);
   }
 
   void _writeField(WriterHelper writer, HeaderFields field) {