avoid NPE in StandardDecryption#finish

[sa-sh]

NullPointerException occurs at com.lowagie.text.pdf.StandardDecryption#finish(StandardDecryption.java:105)
in PDF enctypted with AES while decrypting object of empty stream (cipher is not initialized in this case)

sample pdf object that causes NPE if try to modify (using PDFStamper) encrypted PDF with such object

    41 0 obj
    <</Length 0/Type/Pattern/PatternType 1/PaintType 1/TilingType 2/BBox[0 0 10 10]/XStep 10/YStep 10/Resources 42 0 R>>stream

    endstream
    endobj

stacktrace of error

java.lang.NullPointerException
	at com.lowagie.text.pdf.StandardDecryption.finish(StandardDecryption.java:105)
	at com.lowagie.text.pdf.PdfEncryption.decryptByteArray(PdfEncryption.java:582)
	at com.lowagie.text.pdf.PdfReader.getStreamBytesRaw(PdfReader.java:2355)
	at com.lowagie.text.pdf.PdfReader.getStreamBytesRaw(PdfReader.java:2385)
	at com.lowagie.text.pdf.PRStream.toPdf(PRStream.java:222)
	at com.lowagie.text.pdf.PdfIndirectObject.writeTo(PdfIndirectObject.java:164)
	at com.lowagie.text.pdf.PdfWriter$PdfBody.add(PdfWriter.java:410)
	at com.lowagie.text.pdf.PdfWriter.addToBody(PdfWriter.java:841)
	at com.lowagie.text.pdf.PdfStamperImp.close(PdfStamperImp.java:320)
	at com.lowagie.text.pdf.PdfStamper.close(PdfStamper.java:246)

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