There is at the moment no way of telling what is abstaining and what is apathy.
Abstaining from voting is a deliberate act, much the same as casting a vote is a deliberate act. Simply not voting could be a deliberate vote or it could be apathy.
If there was an option on the form marked abstain that you could tick then I think it would be reasonable to exclude that vote from any requirement to be part of the 50.1% of union members.
Not returning a vote should be counted as a no vote because taking strike action is a deliberate act. If you cannot be bothered to vote, what likelihood you will actively support a strike?