Correct vertex data reading for Unity < 4 meshes

[dv1x3r]

I fixed two issues in MeshHelper:

1. get_channels: bytearray(n) creates n zero bytes not a bit array.
   Just a different behaviour if number is passed as an argument :D
   So I replaced it with bitwise comparison channelMask & (1 << i)

2. read_vertex_data: skip processing if m_VertexCount or m_DataSize is empty.
   For compressed meshes, the m_VertexData.m_VertexCount is 0, and m_VertexData.m_DataSize is empty.
   UnityPy caused a crash in UnityPyBoost.unpack_vertexdata
   AssetStudio handles this gracefully because C# Buffer.BlockCopy silently copies 0 bytes when data is empty.

Sample: Login.unity3d.zip

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
Cell In[14], line 26
     24 file_name = f"{obj.path_id}.{data.m_Name}.obj"
     25 file_path = os.path.join(export_path, env.file.name, "models", file_name)
---> 26 if wavefront_data := data.export():
     27     with open(file_path, "wt", newline="") as f:
     28         f.write(wavefront_data)

File ~/.venv/lib/python3.13/site-packages/UnityPy/classes/legacy_patch/Mesh.py:7, in _Mesh_export(self, format)
      4 def _Mesh_export(self: Mesh, format: str = "obj"):
      5     from ...export.MeshExporter import export_mesh
----> 7     return export_mesh(self, format)

File ~/.venv/lib/python3.13/site-packages/UnityPy/export/MeshExporter.py:13, in export_mesh(m_Mesh, format)
     11 def export_mesh(m_Mesh: Mesh, format: str = "obj") -> str:
     12     if format == "obj":
---> 13         return export_mesh_obj(m_Mesh)
     14     raise NotImplementedError(f"Export format {format} not implemented")

File ~/.venv/lib/python3.13/site-packages/UnityPy/export/MeshExporter.py:19, in export_mesh_obj(mesh, material_names)
     17 def export_mesh_obj(mesh: Mesh, material_names: Optional[List[str]] = None) -> str:
     18     handler = MeshHandler(mesh)
---> 19     handler.process()
     21     m_Mesh = handler
     22     if m_Mesh.m_VertexCount <= 0:

File ~/.venv/lib/python3.13/site-packages/UnityPy/helpers/MeshHelper.py:194, in MeshHandler.process(self)
    188     self.m_IndexBuffer = cast(
    189         List[int],
    190         struct.unpack(f"<{len(raw_indices) // index_size}{char}", raw_indices),
    191     )
    193 if self.version >= (3, 5):
--> 194     self.read_vertex_data(m_Channels, m_Streams)
    196 if isinstance(mesh, Mesh) and self.version >= (2, 6):
    197     self.decompress_compressed_mesh()

File ~/.venv/lib/python3.13/site-packages/UnityPy/helpers/MeshHelper.py:378, in MeshHandler.read_vertex_data(self, m_Channels, m_Streams)
    375 channel_dimension = m_Channel.dimension & 0xF
    377 if UnityPyBoost:
--> 378     componentBytes = UnityPyBoost.unpack_vertexdata(
    379         m_VertexData.m_DataSize,
    380         component_byte_size,
    381         m_VertexCount,
    382         m_Stream.offset,
    383         m_Stream.stride,
    384         m_Channel.offset,
    385         channel_dimension,
    386         swap,
    387     )
    388 else:
    389     componentBytes = bytearray(
    390         m_VertexCount * channel_dimension * component_byte_size
    391     )

ValueError: Vertex data access out of bounds