Improvements to WC_Product_Variable_Data_Store_Custom_Table::sync_managed_variation_stock_status()

[rodrigoprimo]

This PR makes two improvements to the method WC_Product_Variable_Data_Store_Custom_Table::sync_managed_variation_stock_status():

• It fixes a MySQL error when running the method for a variable product with no variations by adding a check to see if the variable product has any children before running the query to update them. This error was happening when running the PHPUnit test WC_Tests_Product_Variable::test_variable_product_auto_stock_status(). Commit 3c58da0
• It adds a check to only update product variable children data if their stock status changed following the same pattern used by the WC core method with the same name. Commit b212ae6