Fix logging crash

Differential Revision: D82666928

Pull Request resolved: pytorch#14388

Author: metascroy

extension/apple/ExecuTorch/Exported/ExecuTorchLog.mm

@@ -90,9 +90,9 @@ - (void)logWithLevel:(ExecuTorchLogLevel)level
     [self->_buffer addObject:@{
       @"level" : @(level),
       @"timestamp" : @(timestamp),
-      @"filename" : filename,
+      @"filename" : filename ?: @"(null)",
       @"line" : @(line),
-      @"message" : message
+      @"message" : message ?: @"(null)"
     }];
   });
   for (id<ExecuTorchLogSink> sink in sinks) {