Check if download_id exists before using #8181

[ashleyfae]

Fixes #8181

Proposed Changes:

1. Check if download_id key exists before using.
2. Only add item to cart if $download_id isn't empty.
3. Don't save edd_add_to_cart() output to a variable, as the variable is never used.

I opted to continue the flow of the function instead of bailing immediately, because that way you'll still get redirected to checkout, even though your cart is empty. I thought this might make it easier to debug a broken URL.

To test:

1. Manually visit a malformed add to cart URL like this: https://yoursite.com/checkout/?edd_action=add_to_cart . Ensure you are redirected to checkout with no items in your cart. Ensure no PHP notices.
2. Manually visit a correctly formed add to cart URL like this: https://yoursite.com/checkout/?edd_action=add_to_cart&download_id=123 (replace 123 with ID of a valid download). Ensure you are redirected to checkout with that item in your cart.

[robincornett]

• tested with incomplete URL (missing download_id)
• tested with valid download_id
• tested with invalid download_id

[LisaCee]

• Replicated error in debug log on release/3.0 with missing download_id

• No error on this branch

• Item added to cart with valid download_id, no error

• No item added to cart with invalid download_id, no error