Ensure that calculateChange() returns zero when both current and previous values are zero.

[techanvil]

Feature Description

At the moment, the calculateChange() function returns null when the previous value is zero, as seen here:

site-kit-wp/assets/js/util/index.js

Lines 87 to 91 in 100ca94

	export function calculateChange( previous, current ) {
	// Prevent divide by zero errors.
	if ( '0' === previous || 0 === previous || Number.isNaN( previous ) ) {
	return null;
	}

However, this is incorrect in the scenario where current is also zero, in which case the change should be returned as 0 rather than null.

This was identified as a fix for a point raised in QA for #6244 (see "Question 2" here), but appears to be a valid fix to make across the board for calculateChange(). However, with this function being used widely in the codebase, due diligence must be applied to confirm this is the case and validate with a good round of testing.

Note: There is a closed PR which can be recreated as the fix for this assuming it's confirmed as above.

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Acceptance criteria

• The ChangeBadge rendered in Key Metric Widgets should show 0% where both, the previous and current values are 0.
• "Change" badges on other existing components such as the Admin Bar, WPDashboard Widget and DataBlocks on the Main and Entity dashboards should NOT be affected by this change.

Implementation Brief

• There is a closed PR which can be recreated as the fix for this assuming it's confirmed as above. The calculateChange() function should return 0 when both the previous and current values are zero.

Test Coverage

• No new tests required.

QA Brief

• Verify the ACs using a site with Zero data, since such a site will have "zero" values for the previous and current periods.

Changelog entry

• Fix bug that could cause zero percent in key metric widgets not to appear.

[wpdarren]

QA Update: ✅

Verified:

• The ChangeBadge rendered in Key Metric Widgets shows 0% where both, the previous and current values are 0.
• The ChangeBadge on other existing components such as the Admin Bar, WPDashboard Widget and DataBlocks on the Main and Entity dashboards are not affected by this change.
• Checked the ChangeBadge on a site that is not zero data to make sure that they continue to appear on the Key metrics widget and other areas of the plugin.