title: empty JaxbSmlPart in newest versions of xlsx4j
body: <p>I hope somebody can give me a hint to solve this problem:
  After a java upgrade to 11 and docx4j upgrade to 11.1.3 (tried 8.1.3 as an alternative with same result) I can't get the content of the SharedStrings part of xlsx files. This happens even with the sample files (VariableReplace or PartsList, e.g.).</p>
  
  <p>Here's the code I used to evaluate the problem. It's taken from the VariableReplace sample file:</p>
  
  <pre><code>SpreadsheetMLPackage opcPackagepkg = SpreadsheetMLPackage.load(new File(inputfilepath));
  System.out.println("\n\nPART TYPE\n\n:" + opcPackagepkg.getParts().get(new PartName("/xl/sharedStrings.xml")).toString());   // gives correct type "org.docx4j.openpackaging.parts.SpreadsheetML.SharedStrings@79518e00"
  JaxbSmlPart smlPart = (JaxbSmlPart)opcPackagepkg.getParts().get(new PartName("/xl/sharedStrings.xml"));
  System.out.println("\n\nBEFORE sml\n:" + smlPart.getXML());    // smlPart.getXML() gives null as result
  
  // and so, of course, the following ends with an exception:
  HashMap&lt;String, String&gt; mappings = new HashMap&lt;String, String&gt;();
  mappings.put("title1", "This is a title");
  smlPart.variableReplace(mappings);   // ends with an NullPointerException
  </code></pre>
  
  <p>I tried several Excel files just to be sure to not have a corrupted one. Everytime same result.
  I also unzipped the Excel files to view the /xl/sharedStrings.xml file. 
  I even tried to use the line</p>
  
  <pre><code>JaxbSmlPart smlPart = (JaxbSmlPart)opcPackagepkg.getParts().get(new PartName("/xl/worksheets/sheet1.xml"));
  </code></pre>
  
  <p>but this gives me the correct result to be a WorkSheetPart (which correctly results in an docx4j exception then because it's not a SharedStrings part).</p>
  
  <p>Any ideas where I'm wrong? Thank you!</p>