add default value for percentileValue

[MGJamJam]

Work:

• adds default percentileValue when percentile is selected so that first query that gets triggered when percentile metric is selected does not fail
• deletes percentileValue once percentile is deselected to not pollute the dashboard.json file.

[Svarozic]

why not simple else , why is there another if ?

Suggested change

	} else {
	if (percentileValue != null) {
	setPercentileValue(undefined);
	}
	}
	} else {
	setPercentileValue(undefined);
	}

[MGJamJam]

My thinking was that if the percentileValue is already null we do not need to update the state because it already would be null/undefined. 🤔 But I guess it would make the code more readable to delete the if as it is really not necessary.