Fix delta calculation when previous and current values are zero

Author: rachelmcr

WooCommerce/Classes/ViewRelated/Dashboard/Factories/StatsDataTextFormatter.swift

@@ -123,8 +123,8 @@ extension StatsDataTextFormatter {
     /// Creates the `DeltaPercentage` for the percent change from the previous `Decimal` value to the current `Decimal` value
     ///
     static func createDeltaPercentage(from previousValue: Decimal?, to currentValue: Decimal?) -> DeltaPercentage {
-        guard let previousValue, let currentValue else {
-            return DeltaPercentage(value: 0) // Missing data: 0% change
+        guard let previousValue, let currentValue, previousValue != currentValue else {
+            return DeltaPercentage(value: 0) // Missing or equal values: 0% change
         }
 
         guard previousValue > 0 else {

WooCommerce/WooCommerceTests/ViewRelated/Dashboard/StatsDataTextFormatterTests.swift

@@ -294,6 +294,19 @@ final class StatsDataTextFormatterTests: XCTestCase {
         XCTAssertEqual(delta.direction, .zero)
     }
 
+    func test_createDeltaPercentage_returns_expected_zero_delta_for_zero_values() {
+        // Given
+        let previousValue: Double = 0
+        let currentValue: Double = 0
+
+        // When
+        let delta = StatsDataTextFormatter.createDeltaPercentage(from: previousValue, to: currentValue)
+
+        // Then
+        XCTAssertEqual(delta.string, "+0%")
+        XCTAssertEqual(delta.direction, .zero)
+    }
+
     func test_createDeltaPercentage_returns_100_percent_change_when_previous_value_is_zero() {
         // Given
         let previousValue: Double = 0