Unsigned signature boxes in the output (#591)

shape 0 | OutputToPDF=True Print=True Visible=True | IsSigned=True HasPaths=False paths=-1 image=THROWS(RadPdfDocumentEditorException) method=TypeText | Text="Signature of SHAPE0."
shape 1 | OutputToPDF=False Print=True Visible=True | IsSigned=True HasPaths=False paths=-1 image=THROWS(RadPdfDocumentEditorException) method=TypeText | Text="Signature of SHAPE1."
shape 2 | OutputToPDF=True Print=False Visible=True | IsSigned=True HasPaths=False paths=-1 image=THROWS(RadPdfDocumentEditorException) method=TypeText | Text="Signature of SHAPE2."
shape 3 | OutputToPDF=False Print=False Visible=True | IsSigned=True HasPaths=False paths=-1 image=THROWS(RadPdfDocumentEditorException) method=TypeText | Text="Signature of SHAPE3."
shape 4 | OutputToPDF=True Print=True Visible=False | IsSigned=True HasPaths=False paths=-1 image=THROWS(RadPdfDocumentEditorException) method=TypeText | Text="Signature of SHAPE4."
shape 5 | OutputToPDF=True Print=True Visible=True | IsSigned=False HasPaths=False paths=-1 image=THROWS(RadPdfDocumentEditorException) method=None | Text=""
exported: 201215 bytes
exported header: ?�me����RadP
exported to: C:\Windows\TEMP\sigout-55644.pdf
re-ingest of the export failed: PDF File Could Not Be Loaded. (0xBFFFFF5B OpenImportBuffer: Invalid file header!)
viewer DocumentID: 55644

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