omit_default breaks IntFlag serialization

[xrun1]

• mashumaro version: 3.11
• Python version: 3.11.6
• Operating System: NixOS

Description

I need to serialize a dataclass that has IntFlag fields and use the omit_default option, but it results in a SyntaxError unless the field is declared without a default.

What I Did

Minimal example:

from dataclasses import dataclass
from enum import IntFlag, auto

from mashumaro.config import BaseConfig
from mashumaro.mixins.dict import DataClassDictMixin


class Flags(IntFlag):
    a = auto()
    

@dataclass
class Data(DataClassDictMixin):
    flags: Flags = Flags.a

    class Config(BaseConfig):
        omit_default = True


print(Data().to_dict())

Result:

Traceback (most recent call last):
  File "/tmp/m.py", line 13, in <module>
    class Data(DataClassDictMixin):
  File "/nix/store/m5i9ys5n65cc01955ngifyyy9nd3nd56-python3.11-mashumaro-3.11/lib/python3.11/site-packages/mashumaro/mixins/dict.py", line 25, in __init_subclass__
    compile_mixin_packer(cls, **builder_params["packer"])
  File "/nix/store/m5i9ys5n65cc01955ngifyyy9nd3nd56-python3.11-mashumaro-3.11/lib/python3.11/site-packages/mashumaro/core/meta/mixin.py", line 29, incompile_mixin_packer
    builder.add_pack_method()
  File "/nix/store/m5i9ys5n65cc01955ngifyyy9nd3nd56-python3.11-mashumaro-3.11/lib/python3.11/site-packages/mashumaro/core/meta/code/builder.py", line1189, in add_pack_method
    self.compile()
  File "/nix/store/m5i9ys5n65cc01955ngifyyy9nd3nd56-python3.11-mashumaro-3.11/lib/python3.11/site-packages/mashumaro/core/meta/code/builder.py", line306, in compile
    exec(code, self.globals, self.__dict__)
  File "<string>", line 4
    if value != <Flags.a: 1>:
                ^
SyntaxError: invalid syntax

[Fatal1ty]

Thank you for spotting this bug!