Fix handling of null in Text::cleanText()

Credits to smulvih2 from DaveChild#55

Author: mdio

src/DaveChild/TextStatistics/Text.php

@@ -20,7 +20,7 @@ public static function cleanText($strText)
     {
 
         // Check for boolean before processing as string
-        if (is_bool($strText)) {
+        if (is_bool($strText) || is_null($strText)) {
             return '';
         }
 

tests/TextStatisticsTest.php

@@ -28,6 +28,7 @@ public function tearDown(): void
     -------------------- */
     public function testCleaning()
     {
+        $this->assertSame('', DaveChild\TextStatistics\Text::cleanText(null));
         $this->assertSame('', DaveChild\TextStatistics\Text::cleanText(false));
         $this->assertSame('There once was a little sausage named Baldrick. and he lived happily ever after.', DaveChild\TextStatistics\Text::cleanText('There once was a little sausage named Baldrick. . . .  And he lived happily ever after.!! !??'));
     }