Permit CSV Delimiter to be Set to Null (#2288)

* Permit CSV Delimiter to be Set to Null

See issue #2287. A 1-character change. The delimiter variable is defined as nullable, and getDelimiter can return null; setDelimiter should follow suit.

* Scrutinizer Inanity

Are you sure the test always returns null?????
Yes, I'm sure, that's why it's part of the test.
Let's see if we can recode it and miss this "problem".

src/PhpSpreadsheet/Reader/Csv.php

@@ -369,7 +369,7 @@ class Csv extends BaseReader
         return $this->delimiter;
     }
 
-    public function setDelimiter(string $delimiter): self
+    public function setDelimiter(?string $delimiter): self
     {
         $this->delimiter = $delimiter;
 

tests/PhpSpreadsheetTests/Reader/Csv/CsvTest.php

@@ -251,4 +251,25 @@ EOF;
             [(version_compare(PHP_VERSION, '7.4') < 0) ? "\x0" : '', ','],
         ];
     }
+
+    /**
+     * This test could be simpler, but Scrutinizer has a minor (and silly) problem.
+     *
+     * @dataProvider providerNull
+     */
+    public function testSetDelimiterNull(?string $setNull): void
+    {
+        $reader = new Csv();
+        $reader->setDelimiter(',');
+        self::assertSame(',', $reader->getDelimiter());
+        $reader->setDelimiter($setNull);
+        self::assertSame($setNull, $reader->getDelimiter());
+    }
+
+    public function providerNull(): array
+    {
+        return [
+            [null],
+        ];
+    }
 }