Revert "Support empty values in gss_key_value_set_desc"

Upon further discussion, the cred_store extension designers are opposed
to supporting empty values for keys.

This reverts commit 2d12ed9.

Author: frozencemetery

gssapi/raw/ext_cred_store.pyx

@@ -83,8 +83,7 @@ cdef gss_key_value_set_desc* c_create_key_value_set(dict values) except NULL:
 
     for (i, (k, v)) in enumerate(values.items()):
         res.elements[i].key = k
-        if v:
-            res.elements[i].value = v
+        res.elements[i].value = v
 
     return res