[python-flask] __init__.py contains NUL characters

Description

When generating for python-flask, swagger_server/__init__.py contains four NUL characters that cause the following error:

% python -m swagger_server            
Traceback (most recent call last):
  File ".../python3.10/runpy.py", line 187, in _run_module_as_main
    mod_name, mod_spec, code = _get_module_details(mod_name, _Error)
  File ".../python3.10/runpy.py", line 146, in _get_module_details
    return _get_module_details(pkg_main_name, error)
  File ".../python3.10/runpy.py", line 110, in _get_module_details
    __import__(pkg_name)
ValueError: source code string cannot contain null bytes

Swagger-codegen version

swagger-codegen: stable 3.0.46 (installed via Homebrew)
OS: Mac OS Ventura 13.4

Swagger declaration file content or url

Any declaration file works.

openapi: '3.0.2'
info:
  title: Test
  version: '1.0'
servers:
  - url: https://api.server.test/v1
paths:
  /method1:
    post:
      summary: Method 1
      responses:
        '200':
          description: OK

Command line used for generation

swagger-codegen generate -i test.yml -l python-flask -o .

Steps to reproduce

Just try to run the app as usual.

Related issues/PRs

Searched; none found.

Suggest a fix/enhancement

TBD

Start by reproducing the issue with the supplied OpenAPI YAML and swagger-codegen generate -i test.yml -l python-flask -o ., then inspect the generated swagger_server/__init__.py and the python-flask generator template that produces it. Done means the generated file contains no NUL characters and python -m swagger_server starts without the reported ValueError.