Add null check for cssNode in stylesheets processor

[FDiskas]

Add null check for cssNode to prevent errors.

potentially fixes #43

What the patch does

• Adds a defensive null check before rewriting style text inside the plugin transform.
• If no style node is found, the plugin returns the original source text instead of attempting to access a missing property.

In practice, this prevents the runtime error:

• Cannot read properties of undefined (reading text)