[16.0] [FIX] membership_prorate: Prevent error when using prorate with variable period without membership_prorate_variable_period module installed

[ByteMeAsap]

This PR aims to solve an unwanted error coming up when using prorates

Steps to reproduce the error:

1. Install membership_prorate and membership_variable_period modules that leads to auto installation of membership_prorate_variable_period module.
2. Uninstall the membership_prorate_variable_period module (Users might do it as they want to use prorates with variable periods)
3. Setup a product with Membership Type set as 'variable' and Prorate enabled
4. Create a invoice with this product, the system would throw an error as

membership_prorate/models/account_move_line.py", line 29, in _prepare_invoice_line_prorate_vals
    if date_invoice < date_from:
TypeError: '<' not supported between instances of 'datetime.date' and 'bool'