Minor fix

For the case of an empty CS value it has been noticed that the number attribute gets allocated to the next tag. Not adding the number attribute when the value is null, seems to solve the problem.

For more details on this issue, take the case of a DICOM file containing the following two tags 
(0008,0008)	ImageType 	DERIVED\SECONDARY\
(0008,0016)	SOPClassUID	1.2.840.10008.5.1.4.1.1.7

The xml conversion for these two is:
    <DicomAttribute keyword="ImageType" tag="00080008" vr="CS">
        <Value number="1">DERIVED</Value>
        <Value number="2">SECONDARY</Value>
    </DicomAttribute>
    <DicomAttribute number="3" keyword="SOPClassUID" tag="00080016" vr="UI">
        <Value number="1">1.2.840.10008.5.1.4.1.1.7</Value>
    </DicomAttribute>

Please note the redundant 'number="3"' associated to the SOPClassUid tag.