fix: aerich migrate crashes without init-db & aerich init-db should create folder after validating app

[hail-pas]

📌 Issue: Two Problems with aerich migrate and init-db Behavior

💥 Problem 1: aerich migrate crashes if init-db was not run

Steps to Reproduce:

1. Run aerich init
2. Do not run aerich init-db
3. Run aerich migrate

Observed Error:

File "/path/to/aerich/migrate.py", line 176, in migrate
    cls.diff_models(last_version, new_version_content)
File "/path/to/aerich/migrate.py", line 390, in diff_models
    old_models.pop(_aerich, None)
AttributeError: 'NoneType' object has no attribute 'pop'

💥 Problem 2: aerich init-db should create folder before validating app

Steps to Reproduce:

1. Add an invalid or non-existent app to the Aerich config
2. Run aerich --app non_existed init-db

Observed Error:

Error: Can't get app named 'non_existed'  # but the migration folder created

[waketzheng]

Hi, @hail-pas
How to reproduce problem 1?
I got: Error: You need to run aerich init-db first to initialize the database. with the following steps:

echo 'from __future__ import annotations

from tortoise import Model, fields

TORTOISE_ORM = {
    "connections": {"default": "sqlite://db.sqlite3"},
    "apps": {"models": {"models": ["models", "aerich.models"]}},
}


class Offer(Model):
    name = fields.CharField(max_length=20)
' > models.py
aerich init -t models.TORTOISE_ORM
aerich migrate

And for problem2, I got:

(venv) mac10.12 in ~/trying/something λ aerich --app not_exists init-db
/Users/mac10.12/trying/something/venv/lib/python3.11/site-packages/tortoise/__init__.py:514: RuntimeWarning: Module "models" has no models
  cls._init_apps(apps_config)
Success creating app migration folder migrations/not_exists
Success generating initial migration file for app "not_exists"
(venv) mac10.12 in ~/trying/something λ ls migrations
not_exists

settings:

TORTOISE_ORM = {
    "connections": {"default": "sqlite://db.sqlite3"},
    "apps": {
        "models": {"models": ["models", "aerich.models"]},
        "not_exists": {"models": ["models"]},
    },
}