fix: Allow creating variables when input starts with dashes in advanced style section

[kof]

Description

We broke this as we introduced a more flexible search to combobox when you add properties

Steps for reproduction

1. advanced panel
2. type --x
3. see you can create that variable right away without adding value

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: 0000)
• updated test cases document
• added tests
• if any new env variables are added, added them to .env file