Ruby 3.3.0 changed the BasicSocket#recv method to return nil instead
of an empty string when used on a closed socket. The netsed tests need
to be adjusted to account for that.

See https://github.com/ruby/ruby/commit/2e4e4c82c282622a94b9499c182a7e52e21a23b4

diff -upr netsed-1.3.orig/test/tc_tcp.rb netsed-1.3/test/tc_tcp.rb
--- netsed-1.3.orig/test/tc_tcp.rb	2024-07-11 20:49:02.453610429 +0200
+++ netsed-1.3/test/tc_tcp.rb	2024-07-11 20:49:31.873686975 +0200
@@ -61,7 +61,7 @@ class TC_TCPTest < Test::Unit::TestCase
   # Check when there is no server
   def test_case_02_NoServer
     datarecv = TCPSingleDataRecv(self.class::CONFIG[:SERVER], LPORT, 100)
-    assert_equal('', datarecv)
+    assert_equal(nil, datarecv)
   end
 
   # Check when the client sends the data