Add Test For NULL=0 (#2607)

Fix #2523. This isn't actually a fix; the problem was reported and confirmed for 1.21, but had already been fixed in master (and remains fixed in 1.22). This PR just adds a unit test for the original problem.

Author: oleibman

tests/PhpSpreadsheetTests/Calculation/NullEqualsZeroTest.php

@@ -0,0 +1,32 @@
+<?php
+
+namespace PhpOffice\PhpSpreadsheetTests\Calculation;
+
+use PhpOffice\PhpSpreadsheet\Spreadsheet;
+use PHPUnit\Framework\TestCase;
+
+class NullEqualsZeroTest extends TestCase
+{
+    public function testNullEqualsZero(): void
+    {
+        // Confirm that NULL<>0 returns false
+        $spreadsheet = new Spreadsheet();
+        $sheet = $spreadsheet->getActiveSheet();
+        $sheet->fromArray([
+            ['Item', 'QTY', 'RATE', 'RATE', 'Total'],
+            ['Bricks', 1000, 'Each', 0.55, '=IF(B2<>0,B2*D2,"INCL.")'],
+            ['Cement', null, null, null, '=IF(B3<>0,B3*D3,"INCL.")'],
+            ['Labour', 10, 'Hour', 45.00, '=IF(B4<>0,B4*D4,"INCL.")'],
+        ]);
+        $sheet->setCellValue('A6', 'Total');
+        $sheet->setCellValue('E6', '=SUM(E1:E5)');
+        self::assertEquals(550.00, $sheet->getCell('E2')->getCalculatedValue());
+        self::assertSame('INCL.', $sheet->getCell('E3')->getCalculatedValue());
+        self::assertEquals(450.00, $sheet->getCell('E4')->getCalculatedValue());
+        self::assertEquals(1000.00, $sheet->getCell('E6')->getCalculatedValue());
+
+        $sheet->setCellValue('Z1', '=Z2=0');
+        self::assertTrue($sheet->getCell('Z1')->getCalculatedValue());
+        $spreadsheet->disconnectWorksheets();
+    }
+}