PDFBOX-5814: warning only for non empty files, as suggested by Anthon…

…y Brunellière

git-svn-id: https://svn.apache.org/repos/asf/pdfbox/trunk@1917534 13f79535-47bb-0310-9956-ffa450edef68

pdfbox/src/main/java/org/apache/pdfbox/pdmodel/PDDocument.java

@@ -979,7 +979,7 @@ public void save(OutputStream output) throws IOException
      */
     public void save(File file, CompressParameters compressParameters) throws IOException
     {
-        if (file.exists())
+        if (file.exists() && file.length() > 0)
         {
             LOG.warn(
                     "You are overwriting the existing file {}, this will produce a corrupted file if you're also reading from it",