KDC-REQ is defined as
 * KDC-REQ ::= SEQUENCE {
 * -- NOTE: first tag is [1], not [0]
 * pvno [1] INTEGER (5) ,
 * msg-type [2] INTEGER (10 -- AS -- | 12 -- TGS --),
 * padata [3] SEQUENCE OF PA-DATA OPTIONAL
 * -- NOTE: not empty --,
 * req-body [4] KDC-REQ-BODY
 * }

However, in sun.security.krb5.internal.KDCReq.init(DerValue encoding, int req_type), we have

...
147 subDer = der.getData().getDerValue();
        if ((subDer.getTag() & 0x01F) == 0x03) {
            ... // Parsing padata
        } else {
            pAData = null;
        }
164 subDer = der.getData().getDerValue();
        if ((subDer.getTag() & 0x01F) == 0x04) {
... // Parsing req-body

Here if padata is missing (it's marked as OPTIONAL in definition), subDer read at line 147 is already req-body, so reading on line 164 is redundant and would fail.

The priority is marked as 5-Very Low because JDK is not used at writing KDC, so this method might never be used.