Fix - Sale price effective date

[vinkmeta]

Changes proposed in this Pull Request:

When User doesn't provide any value for sale price start/end dates then by default we set Start Date as 1970-01-29 and End Date as 2038-01-17 which is incorrect. Instead we should not pass any value for sale price effective date, start date and end date when sale price is not populated.

Screenshots:

Screenshots from Meta Commerce Manager

Before:

After:

Detailed test instructions:

1. Unit test - ./vendor/bin/phpunit --filter test_sale_price_and_effective_date
2. This change only impacts sale_price_effective_date and no other product fields or user experience
3. Earlier default value for sale_price_effective_date use to be '1970-01-29T00:00+00:00/2038-01-17T23:59+00:00' when no value was set for $sale_price_start_date and $sale_price_end_date by the plugin user but now this will set as empty in that case.

Changelog entry

Fix - Sale price effective date

[rawdreeg]

I have tested these changes; they effectively fix the issue. LGTM!