fix(repl): avoid seg fault on ctrl-d

[maedoc]

(hi! tahnks for the very nice project 🤗 )

el_gets on my macos sequoia 15.5 returns NULL when typing Ctrl-D to exit the reply. This is a tiny change to parse a NULL as if user entered quit.

For reference here is backtrace,

--- version 0.1.21
Manta IS NOT connected.
Type 'helpall' to get a list of all built-in functions.
Type 'quit' to quit.
sapf> ^D
💣 Program crashed: Bad pointer dereference at 0x0000000000000000

Thread 3 crashed:

 0 0x000000018b86b320 _platform_strncmp$VARIANT$Base + 176 in libsystem_platform.dylib
 1 Thread::compile(char const*, P<Fun>&, bool) + 432 in sapf at /Users/tng/src/sapf/src/VM.cpp:675:2

   673│         P<Code> code;
   674│         setParseString(inString);
   675│         getLine();                                                                                                                 
      │  ▲
   676│         bool ok = parseElems(th, code);
   677│         if (!ok || !code) {

 2 Thread::repl(__sFILE*, char const*) + 1000 in sapf at /Users/tng/src/sapf/src/VM.cpp:359:9

   357│                         {
   358│                                 P<Fun> compiledFun;
   359│                                 if (compile(NULL, compiledFun, true)) {                                                            
      │         ▲
   360│                                 // EVAL
   361│                                         compiledFun->runREPL(th);

 3 __main_block_invoke_2 + 68 in sapf at /Users/tng/src/sapf/src/main.cpp:201:6

   199│         
   200│         dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
   201│                 th.repl(stdin, vm.log_file);                                                                                       
      │      ▲
   202│                 exit(0);
   203│         });

 4 0x000000018b676b2c _dispatch_call_block_and_release + 32 in libdispatch.dylib
 5 0x000000018b69085c _dispatch_client_callout + 16 in libdispatch.dylib
 6 0x000000018b6ac840 _dispatch_queue_override_invoke.cold.3 + 32 in libdispatch.dylib
 7 0x000000018b67b278 _dispatch_queue_override_invoke + 848 in libdispatch.dylib
 8 0x000000018b688e30 _dispatch_root_queue_drain + 364 in libdispatch.dylib
 9 0x000000018b6895d4 _dispatch_worker_thread2 + 156 in libdispatch.dylib
10 0x000000018b82ae28 _pthread_wqthread + 232 in libsystem_pthread.dylib