Resolve Issue #2833, NULL value handling in PRODUCT() Excel function

src/PhpSpreadsheet/Calculation/MathTrig/Operations.php

@@ -108,7 +108,7 @@ class Operations
         // Loop through arguments
         foreach (Functions::flattenArray($args) as $arg) {
             // Is it a numeric value?
-            if (is_numeric($arg)) {
+            if (is_numeric($arg) || $arg === null) {
                 if ($returnValue === null) {
                     $returnValue = $arg;
                 } else {

tests/data/Calculation/MathTrig/PRODUCT.php

@@ -48,6 +48,18 @@ return [
         -6.7800000000000002,
         -2,
     ],
+    [
+        0,
+        12.5,
+        null,
+        2.5,
+    ],
+    [
+        0,
+        12.5,
+        2.5,
+        null,
+    ],
     ['#VALUE!', 1, 'y', 3],
     [6, 1, '2', 3],
 ];