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 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:
```xml
    <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.