Permit CSV Delimiter to be Set to Null

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

Author: oleibman

src/PhpSpreadsheet/Reader/Csv.php

@@ -369,7 +369,7 @@ public function getDelimiter(): ?string
         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 +251,13 @@ public function providerEscapes(): array
             [(version_compare(PHP_VERSION, '7.4') < 0) ? "\x0" : '', ','],
         ];
     }
+
+    public function testSetDelimiterNull(): void
+    {
+        $reader = new Csv();
+        $reader->setDelimiter(',');
+        self::assertSame(',', $reader->getDelimiter());
+        $reader->setDelimiter(null);
+        self::assertNull($reader->getDelimiter());
+    }
 }