Fix empty shared string bug

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

src/Spout/Reader/Helper/XLSX/SharedStringsHelper.php

@@ -257,7 +257,7 @@ class SharedStringsHelper
             $sharedString = $this->unescapeLineFeed($escapedSharedString);
         }
 
-        if (!$sharedString) {
+        if ($sharedString === null) {
             throw new SharedStringNotFoundException("Shared string not found for index: $sharedStringIndex");
         }
 

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
      */