Some default settings may be missing for new activations

[tomalec]

Describe the bug:

I didn't find any specific bug, but I see a chance for such :)
The review of #400 revealed that the maybe_set_defaults function introduced in 2.0.0 breaks the behavior of WC_Integration::init_settings.

If the extension (2.0.0+) is newly activated - meaning there were no settings yet. It saves the settings with a single property set 'ga_product_identifier' => 'product_id'. Then, when the WC_Google_Analytics constructor is called, the `init_settings, which is supposed to set all the defaults to what's defined in the form fields, does nothing, as it finds that the settings object is already set.

Steps to reproduce:

1. Deactivate, uninstall GA4W.
2. Remove woocommerce_google_analytics_settings entry from wp_options
3. Install and activate using CLI

wp plugin install woocommerce-google-analytics-integration
wp plugin activate  woocommerce-google-analytics-integration

4. Visit any page as a guest.

Expected behavior:

woocommerce_google_analytics_settings settings should contain all the defaults

Actual behavior:

woocommerce_google_analytics_settings has only ga_product_identifier set.

Additional details:

Maybe it's not a bug, but the desired effect, but I find it confusing and bug-prone to have two sets of defaults.