Fix empty shared string bug

Replaced !$sharedString by $sharedString === null to avoid the case
when $sharedString = ''

tests/Spout/Reader/XLSXTest.php

@@ -140,6 +140,19 @@ class XLSXTest extends \PHPUnit_Framework_TestCase
         $this->assertEquals($expectedRows, $allRows);
     }
 
+    /**
+     * @return void
+     */
+    public function testReadShouldSupportEmptySharedString()
+    {
+        $allRows = $this->getAllRowsForFile('sheet_with_empty_shared_string.xlsx');
+
+        $expectedRows = [
+            ['s1--A1', '', 's1--C1'],
+        ];
+        $this->assertEquals($expectedRows, $allRows);
+    }
+
     /**
      * @return void
      */