TelnetServer: Ignore null and \n when parsing

This fixes issues with carriage return sequences.

Before, using <CR><NUL> as the return sequence wouldn't work at all,
and when using <CR><LF> there was an extra newline after every newline.

After this patch, the behaviour should be closer to the Telnet RFC.

Userland/Services/TelnetServer/Parser.cpp

@@ -24,6 +24,10 @@ void Parser::write(StringView data)
                 if (on_data)
                     on_data("\n");
                 break;
+            case '\0':
+            case '\n':
+                // Ignore.
+                break;
             default:
                 if (on_data)
                     on_data(StringView(&ch, 1));