fix: schema defaulting to None instead of "public" in postgres database integration

[dusklight00]

Describe the bug
When schema is not described in data_sources of config.yaml, then by default schema takes the value None, which is when passed to create_engine function from sqlalchemy, which is not properly handled and that creates a poor connection and tables are not loaded.

Expected behavior
There should be a proper condition to handle if schema is set to none instead of directly passing it to create_engine function.

To Reproduce
Step 1: Setup sample SQL given in https://docs.datachecks.io/getting_started/ in PostgresSQL
Step 2: Copy paste the config.yaml as given by the documentation, and customize it according to your database name, mine is following.

data_sources:
  - name: postgres
    type: postgres
    connection:
      host: 127.0.0.1
      port: 5432
      username: postgres
      password: password
      database: postgres
metrics:
  - name: count_of_products
    metric_type: row_count
    resource: postgres.products
    validation:
      threshold: "> 0 & < 1000"
  - name: max_product_price_in_india
    metric_type: max
    resource: postgres.products.price
    filters:
      where: "country_code = 'IN'"
    validation:
      threshold: "< 190"

Step 3: Run the project using the command poetry run datachecks inspect -C config.yaml

Screenshots
When schema is set to none

When schema is made public

Logs and additional context

[subhankarb]

Fixed with #153