Undefined array key "weight" commerce_flat_rate.module

Problem/Motivation

Not sure if this is a PHP 8.0 thing or what caused it, but flat rate definitions do not always have a "weight" value, so the following warning gets thrown:

Undefined array key "weight" commerce_flat_rate.module:202

Proposed resolution

If the "weight" element isn't defined in the $service, default to 0.

Remaining tasks

User interface changes

n/a

API changes

n/a

Data model changes

n/a