[llvm-nm] Fix msan error in llvm-nm/wasm/weak-symbols.yaml.test (#65538)

This happened because we had a section

```
- Index:           3
         Kind:            DATA
         Name:            weak_import_data
         Flags:           [ BINDING_WEAK, UNDEFINED ]
```

Which does not have size. We managed to reproduce it by building llvm
under msan with libcxx as a standard library and debug mode with
-D_LIBCPP_DEBUG_STRICT_WEAK_ORDERING_CHECK. It called comp(a, a) and
full tie detected uninitialized memory

This started to happen after https://reviews.llvm.org/D158799