Space(s) only as custom filter URL value is considered valid (AdGuard Settings)

### ɪꜱꜱᴜᴇ ᴄʜᴀɴɢᴇʟᴏɢ
#### April 20, 2023
- ➕ Added a screenshot in `Steps to reproduce`.
- ➕ Added an animated GIF regarding an alternative possibility in `Expected behavior`.
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##### 💻 Tested and reproducible on both:
- Version `4.1.53` on Firefox `112.0` — Win 10 Pro 22H2
- Version `4.1.55` on Edge `112.0.1722.48` — Win 10 Pro 22H2
 
# 👋 Steps to reproduce

1. Go to `AdGuard Settings` > Filters > Custom > then click on the `Add custom filter` button.
2. [📸 [ꜱᴄʀᴇᴇɴꜱʜᴏᴛ](https://user-images.githubusercontent.com/4764956/233382151-2f73b665-bb10-4590-9106-99e875d22737.png)] Type one space _or more_ in the URL field and click on the `Next` button.

# Actual behavior

It's considered valid! _(Filter will be added with "1 rule" listed in the pre-validation summary.)_

📷 Current result …

 


# Expected behavior

_Unless_ this is intended for quick testing purposes (noop filter), it should not be I would say. **AdGuard for Windows** doesn't permit that.

📷 Expected result …

 

Or, just like in the [MV3](https://github.com/AdguardTeam/AdGuardMV3) experimental version, disable the validation button by default and when this situation occurs:

^{↪️ Makes sense too, if not more.}