Please describe the problem:
The Patient Sex (0010,0040) attribute is incorrect.  Patient Sex (0010,0040) is
a type 2 attribute.  This means that when the correct value is not known, the
attribute tag is present but the value field shall be of zero length.

This might be fixable by just changing the string from "?" to "", but I didn't
track through the various subroutines involved to see whether they handle
encoding null strings correctly.

Steps to reproduce:
1.  Save an image as a DICOM file
2. Examine the file.  Patient Sex (0010,0040) is present as "?", which is an
illegal value.
3. 


Actual results:


Expected results:
The Patient Sex (0010,0040) should have a zero length value field.

Does this happen every time?
Yes

Other information:
I have not run the resulting objects through a DICOM validator.  I just looked
at the file with a hex editor.  There might be other problems.  

The GIMP does not comply with the full set of DICOM media storage requirements,
but it never claims compliance.  There might be no need.  Other programs can do
this separately from GIMP.  One side effect of this is the need for the file
reader to guess the transfer syntax.  The guess logic used is usually right, so
that is a minor issue.  Guessing is needed anyhow to deal with other systems
that also do not comply.