Update dlx-json-converter.js

If the DLxJsonConverter is instantiated with a specification that based on a specificationData of a filteredPacketFieldSpecs the packetField.packed is undefined if a filteredPacketFieldSpecs packet contains a invalid packetId.


Example (correct):
filteredPacketFieldSpecs = [{ 
    filteredPacketFieldId: 'someId',
    packetId: '01_0010_7E11_10_0100', // packetId = headerId with protocol version, I think... 
    fieldId: '000_2_0',
    name: 'Sensor 1',
    type: 'Number_0_1_DegreesCelsius' 
}]

Example (wrong):
filteredPacketFieldSpecs = [{ 
    filteredPacketFieldId: 'someId',
    packetId: '01_0010_7E11_0100',  // only the headerID!
    fieldId: '000_2_0',
    name: 'Sensor 1',
    type: 'Number_0_1_DegreesCelsius' 
}]