Since (I think) the last update from 2.104 to 2.105 something has changed in validation of integer fields resulting in the 'discard builds' section of a freestyle job showing validation warning 'Not a positive integer'.

Empty values are (and always have been) stored as -1.

<logRotator class="hudson.tasks.LogRotator">
  <daysToKeep>-1</daysToKeep>
  <numToKeep>20</numToKeep>
  <artifactDaysToKeep>-1</artifactDaysToKeep>
  <artifactNumToKeep>-1</artifactNumToKeep>
</logRotator>

Changing the value to a positive integer clears the validation warning, but emptying it returns the warning.