fix: Skip empty combo class when pasting from webflow

[kof]

Description

Skipping unnecessary combo class

Steps for reproduction

1. in webflow, add element with class .a, define color
2. add element with class .b, define font size
3. add element with class .a.b don't define anything
4. paste .a.b into webstudio - see .a .b tokens are added, but no .a.b

Code Review

• hi @kof, I need you to do
  • conceptual review (architecture, feature-correctness)
  • detailed review (read every line)
  • test it on preview

Before requesting a review

• made a self-review
• added inline comments where things may be not obvious (the "why", not "what")

Before merging

• tested locally and on preview environment (preview dev login: 5de6)
• updated test cases document
• added tests
• if any new env variables are added, added them to .env file