Keep IndexableText null if it's an empty string

The `Validate()` method is correctly setting it to null, but then the
`UPDATE` SQL statement is triggering another call to
`GetIndexableText()`. Without this change, empty strings are being
written to the `IndexableText` column.

lib/Ebook.php

@@ -729,6 +729,10 @@ final class Ebook{
 			}
 
 			$this->_IndexableText = Formatter::RemoveDiacriticsAndNonalphanumerics($this->_IndexableText);
+
+			if($this->_IndexableText == ''){
+				$this->_IndexableText = null;
+			}
 		}
 
 		return $this->_IndexableText;