Why is json "truncated" for enumerations if externalValue="" ?

I spent some time this morning trying to figure out why an OSLC library I use began complaining about a null result on some  enumeration literal or another.   I figured out "why" but would like to understand why the json output will NOT contain literals which have externalValue="" on their definition ?  [  I guess because they could not be distinguished from one another,otherwise ]

That is, if say priority has the following:

<enumeration attributeTypeId="priority">
                    <literal default="true" externalValue="" icon="processattachment:/enumeration/unassigned.gif" id="priority.literal.l01" name="Unassigned"/>
                    <literal externalValue="" icon="processattachment:/enumeration/low.gif" id="priority.literal.l02" name="Low"/>
                    <literal externalValue="" icon="processattachment:/enumeration/medium.gif" id="priority.literal.l07" name="Medium"/>
                    <literal externalValue="" icon="processattachment:/enumeration/high.gif" id="priority.literal.l11" name="High"/>
                </enumeration>

Doing a REST call to get it (https://server:port/CONTEXT/oslc/enumerations/PROJECT_UUID/priority.json  gives this:

  [
       {
           "rdf:resource": "https://server:port/CONTEXT/oslc/enumerations/PROJECT_UUID/priority/priority.literal.l01",
           "dc:title": "Unassigned"
       }
    ]

I have worked around this by setting the externalValue == id.     Is removing the externalValue="" also an acceptable course ?

I followed the above trail thanks to https://jazz.net/library/article/1002#enumerations

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