Applies a margin-bottom to attribute labels

Created by: naps62

P.S.: Even with the known issues I present below, I still prefer this over the current version, which is why I chose to propose this. No hard feelings if it doesn't go through 😄

Why:

• When a nil attribute exists, the data element takes no height, which results in the row being too close to the next attribute.

This change addresses the need by:

• The spacing between each attribute was previously given by the attribute-data field. But when it has no height, that margin gets swallowed by the label. Applying the same margin to it helps to fix this

Known issues:

• $spacing-base is measured in ems. And since the label has a different font-size, this results in a slightly different margin, which will be noticeable in the same cases as in the original issue, but I guess a different margin is better than no margin.
• When the label spans multiple lines, an even larger spacing gets created

Before:

After: