fix rendering of xslf shapes with no/external blip part

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1861958 13f79535-47bb-0310-9956-ffa450edef68

src/java/org/apache/poi/sl/draw/DrawPaint.java

         assert(graphics != null);
 
         final String contentType = fill.getContentType();
+        if (contentType == null || contentType.isEmpty()) {
+            return TRANSPARENT;
+        }
 
         ImageRenderer renderer = DrawPictureShape.getImageRenderer(graphics, contentType);
 

src/ooxml/java/org/apache/poi/xslf/usermodel/XSLFShape.java

 
             @Override
             public String getContentType() {
+                if (blip == null || !blip.isSetEmbed() || blip.getEmbed().isEmpty()) {
+                    return null;
+                }
                 /* TOOD: map content-type */
                 return getPart().getContentType();
             }