Skip non-product line items in "Sum quantities for products sharing a display"

Once a line item of a non-product line item type is added to the cart (e.g. a shipping line item), evaluation of the "Sum quantities for products sharing a display" action fails and the price is no longer calculated correctly.

Relevant extract from rules debug log:

• 0 ms Rule Override price with price table fires.
• 0.076 ms Evaluating the action commerce_price_table_sum_quantities. [edit]
• 12.31 ms Unable to get a data value. Error: Unknown data property commerce_product.
• 2281.182 ms Unable to evaluate action commerce_price_table_sum_quantities. [edit]
• 2281.819 ms Unable to get variable combined_product_sum, it is not defined.
• 2282.299 ms Unable to evaluate action commerce_price_table_set_price. [edit]
• 2282.339 ms Rule Override price with price table has fired.

Steps to reproduce:

• Make sure, that the cart is displayed on the review page of the checkout process.
• Create a new order for a product and a quantity, so that an alternative sales price is used.
• Make sure, that a line item of a non-product line-item type is added to the order before you reach the review page, e.g. a shipping line item.

Actual result

The "Sum quantities for products sharing a display" action will fail to evaluate and thus, the price shown in the cart on the checkout review page is not calculated correctly.

Expected result

The "Sum quantities for products sharing a display" action is evaluated. The price shown in the cart on the checkout review page is calculated correctly.

Proposed fix

Skip non-product line items in "Sum quantities for products sharing a display" action.