At times you are going to come across the question, can I use Dark hole with no monsters on the field or can I Heavy Storm the Field with no Spell/Traps? Simple and Clean, no.

When you activate a card, there must be possible "targets" on the Field, Graveyard, Hand, or Deck.

[Ex. 1] You cannot activate Heavy Storm/Dark Hole without destroying anything on the field.

[Ex. 2] You cannot activate E-Emergency Call(or any searcher) if you have no HEROs(or any correct targets) left in the deck

[Ex. 3] You cannot activate Monster Reincarnation, or any card that targets a card(s) in the graveyard, if you have no correct targets in the grave in the first place.

[Ex. 4] You cannot activate Upstart Goblin if there are no more cards in your deck.

Just a plain and simple list of things you can do, wasn't expecting it to be long, but if you have any questions let me know.