PluginException when trying to add a Shipping method after adding a package type that has an empty weight

Problem/Motivation

When trying to add a Shipping method after configuring a package type that has an empty weight (e.g 0g), I got a PluginException ("The package type "commerce_package_type"... property "weight" must have a "number" key.)

I debugged it, and it seems to be due to the empty() check performed in the PackageTypeManager class.

      foreach (['number', 'unit'] as $weight_property) {
        if (empty($definition['weight'][$weight_property])) {
        }
     }

Proposed resolution

Replace the empty() call by !isset().

Remaining tasks

Propose a patch.