Refactor: Remove unused FlatFilePos::SetNull

src/test/flatfile_tests.cpp

@@ -23,6 +23,9 @@ BOOST_AUTO_TEST_CASE(flatfile_filename)
 
     FlatFileSeq seq2(data_dir / "a", "b", 16 * 1024);
     BOOST_CHECK_EQUAL(seq2.FileName(pos), data_dir / "a" / "b00456.dat");
+
+    // Check default constructor IsNull
+    assert(FlatFilePos{}.IsNull());
 }
 
 BOOST_AUTO_TEST_CASE(flatfile_open)

src/test/fuzz/flatfile.cpp

@@ -25,6 +25,4 @@ FUZZ_TARGET(flatfile)
         assert((*flat_file_pos == *another_flat_file_pos) != (*flat_file_pos != *another_flat_file_pos));
     }
     (void)flat_file_pos->ToString();
-    flat_file_pos->SetNull();
-    assert(flat_file_pos->IsNull());
 }