I think these were missed in MoarVM#1803.

This fixes the three compiler warnings like the example below:
```
src/io/asyncsocketudp.c:467:57: warning: missing initializer for field ‘mvm_open_mode’ of ‘MVMIOIntrospection’ [-Wmissing-field-initializers]
  467 |                                                         socket_handle };
      |                                                         ^~~~~~~~~~~~~
In file included from src/moar.h:279,
                 from src/io/asyncsocketudp.c:1:
src/io/io.h:86:16: note: ‘mvm_open_mode’ declared here
   86 |     MVMint64 (*mvm_open_mode) (MVMThreadContext *tc, MVMOSHandle *h);
      |                ^~~~~~~~~~~~~
```

I believe putting NULL is safe because of the check here
https://github.com/MoarVM/MoarVM/blob/main/src/disp/syscall.c#L1277.