reimplement and fix unit tests in the largest series product feature

Author: xinri

exercises/practice/largest-series-product/.meta/config.json

@@ -22,7 +22,8 @@
     "Smarticles101",
     "sonapraneeth-a",
     "sshine",
-    "Zaldrick"
+    "Zaldrick",
+    "Xinri"
   ],
   "files": {
     "solution": [

exercises/practice/largest-series-product/src/test/java/LargestSeriesProductCalculatorTest.java

@@ -130,12 +130,35 @@ public void testSeriesLengthLongerThanLengthOfStringToTestIsRejected() {
     @Disabled("Remove to run test")
     @Test
     @DisplayName("reports 1 for empty string and empty product (0 span)")
-    public void testEmptyStringToSearchAndSeriesOfNonZeroLengthIsRejected() {
+    public void testEmptyStringToSearchAndEmptyProduct() {
+        LargestSeriesProductCalculator calculator = new LargestSeriesProductCalculator("");
+
+        long actualProduct = calculator.calculateLargestProductForSeriesLength(0);
+
+        assertThat(actualProduct).isEqualTo(1);
+    }
+
+    @Disabled("Remove to run test")
+    @Test
+    @DisplayName("reports 1 for nonempty string and empty product (0 span)")
+    public void testNonEmptyStringToSearchAndEmptyProduct() {
+        LargestSeriesProductCalculator calculator = new LargestSeriesProductCalculator("123");
+
+        long actualProduct = calculator.calculateLargestProductForSeriesLength(0);
+
+        assertThat(actualProduct).isEqualTo(1);
+    }
+
+    @Disabled("Remove to run test")
+    @Test
+    @DisplayName("rejects empty string and nonzero span")
+    public void testEmptyStringAndNonZeroSpanIsRejected() {
         LargestSeriesProductCalculator calculator = new LargestSeriesProductCalculator("");
 
         assertThatExceptionOfType(IllegalArgumentException.class)
                 .isThrownBy(() -> calculator.calculateLargestProductForSeriesLength(1))
                 .withMessage("Series length must be less than or equal to the length of the string to search.");
+
     }
 
     @Disabled("Remove to run test")