[yaml2obj][MachO] - Fix PubName/PubType handling.

`PubName` and `PubType` are optional fields since D80722.

They are defined as:
  Optional<PubSection> PubNames;
  Optional<PubSection> PubTypes;

And initialized in the following way:
  IO.mapOptional("debug_pubnames", DWARF.PubNames);
  IO.mapOptional("debug_pubtypes", DWARF.PubTypes);

But problem is that because of the issue in `YAMLTraits.cpp`,
when there are no `debug_pubnames`/`debug_pubtypes` keys in a YAML description,
they are not initialized to `Optional::None` as the code expects, but they
are initialized to default `PubSection()` instances.

Because of this, the `if` condition in the following code is always true:

if (Obj.DWARF.PubNames)
  Err = DWARFYAML::emitPubSection(OS, *Obj.DWARF.PubNames,
                                  Obj.IsLittleEndian);

What means `emitPubSection` is always called and it writes few values.

This patch fixes the issue. I've reduced `sizeofcmds` by size of data
previously written because of this bug.

Differential revision: https://reviews.llvm.org/D81686