[form-layout] updateStyles does not set CSS custom properties when custom-style is not imported

[web-padawan]

Two tests started to fail after I moved Lumo and Material to monorepo with removing custom-style:

packages/vaadin-form-layout/test/form-item.test.js:

 ❌ form-item > CSS properties > should apply label-width
      AssertionError: expected '112px' to equal '100px'
      + expected - actual
      
      -112px
      +100px
      
      at o.<anonymous> (packages/vaadin-form-layout/test/form-item.test.js:81:55)

 ❌ form-item > CSS properties > should apply label-spacing
      AssertionError: expected '16px' to equal '8px'
      + expected - actual
      
      -16px
      +8px
      
      at o.<anonymous> (packages/vaadin-form-layout/test/form-item.test.js:97:57)

https://github.com/vaadin/vaadin-web-components/runs/2020100700

When debugging, I noticed the custom CSS property is not set after calling updateStyles

When adding import for custom-style, the tests are passing: e562d20

[web-padawan]

The method and related tests were removed in Vaadin 24, see #4993. Closing.