If you're freely giving away money (either in the form of null transactions or putting money into mining) just to secure value that you're holding, you've clearly mispriced the asset you're trying to secure!
If you put money in a savings account with interest below inflation, just to secure the value that you're holding, did you misprice the value? Or for example if you buy gold and then pay someone to watch it for you, does that mean you mispriced it?
Null transactions require mining. The question is if nobody wants to mine who will mine? Old mining chips are already so cheap they cost less than a cheap bitcoin transaction. It would not cost much to mine just to preserve the value of your holdings.
The point of submitting the transaction would be to incentivize mining by posting a bounty for it, in the form of a transaction fee. It would be just like paying other people to mine at a loss on your behalf.
You could also send null transactions which do nothing but pay a transaction fee