Steps to reproduce the issue

Go to System -> Global Configuration -> Smart Search -> Index

Here you can input negative weight multipliers.

Expected result

After inputting negative weight multipliers and trying to save said changes, get a popup/message notifying you that negative values are prohibited.

Actual result

You can save negative values for weight multipliers.
This will result in an error when trying to index your data with the new multipliers.

System information (as much as possible)

This has been tested on Joomla 4.0 dev and Firefox 91.0.2 64bit with ~1300 articles and Sample blog data installed.

Additional comments

This option makes sense for values from 0-n but values below 0 should be prohibited.