PushHeaderInstructions contains incomplete Ethernet object

PushHeaderInstructions uses Ethernet object only to hold ethernet type.

ethernetType field should be checked as part of this instruction's equality test, but it is not possible, since
Ethernet object with only ethernet type set is a broken instance, which cannot be compared against another Ethernet instance using `equals()`. (Will result in NPE.)

// Ethernet class allowing creation of incomplete instance is probably another separate design flaw, which should be addressed at some point.