Loki: Fix showing of correct line limit in options

[ivanahuckova]

Fixes: #69555

In this PR we simplifying the logic of preprocessMaxLines and we are returning:

• undefined if value is:
  • empty string
  • non-numeric string (e.g. e)
  • negative number
• actual number if it is valid number

This also fixes showing of correct log lines in the Query options as we have there query.maxLines ?? maxLines, which is correct, but it wasn't correctly working with NaN.

How to test:

1. make sure your loki datasource has a configured line limit
2. go to explore, run a query
3. adjust the line-limit in the query editor, run the query
4. now clear the value of the line-limit field in the query editor, run the query
5. you should see correctly displayed configured log line limit from

lineLimit.mov

[github-actions]

Backend code coverage report for PR #69572
No changes

[github-actions]

Frontend code coverage report for PR #69572

Plugin	Main	PR	Difference
loki	84.98%	85.22%	.24%

[gabor]

LGTM 👍