The timing on the 2 effects of "Evil Thorn" (inflicting damage and Special Summoning) are simultaneous. The effect ends after you have done both. If you have no "Evil Thorns" in your Deck, you can still Tribute "Evil Thorn" to inflict 300 damage.
Tributing "Evil Thorn" is a cost to activate its effect.
If you activate the effect of "Evil Thorn", inflicting damage is mandatory but Special Summoning is optional.
If you can't Special Summon, due to a card effect, you can still Tribute "Evil Thorn" to inflict damage.
The timing on the 2 effects of "Evil Thorn" (inflicting damage and Special Summoning) are simultaneous. The effect ends after you have done both.
If you have no copies of "Evil Thorn" in your Deck, you can still Tribute "Evil Thorn" to inflict 300 damage.
The effects of the Special Summoned copies of "Evil Thorn" are negated as long as they remain face-up on the field.