For spells with multiple attack rolls, what happens if there are no targets left?

Question

When are targets picked for spells with multiple "missiles"? tells us that you can (probably) resolve each "beam" or "ray" of, say, eldritch blast or scorching ray one after the other.

Assuming this is true, what happens in the following scenario:

• One enemy left, has low HP;
• PC casts one of the aforementioned spells and hits with their first "beam"/"ray";
• First beam kills that enemy (to revolve any ambiguity about enemy death saving throws, let's assume the PC rolls maximum damage and it kills the enemy outright by exceeding it's maximum HP; 1d10 or 2d6 could certainly do this to a goblin on less than half health);
• What happens to any remaining beams?

Eldritch blast says:

A beam of crackling energy streaks toward a creature within range.

And scorching ray says:

You create three rays of fire and hurl them at targets within range. You can hurl them at one target or several.

Can you choose to not fire any remaining "beams"/"rays"? Can you just fire them at the floor (seems unlikely for eldritch blast since you can only target a creature)? Can you continue firing at the dead enemy anyway (again, unlikely for eldritch blast, since a corpse is not a creature)? Do you have to fire at an ally, or whoever the nearest creature is (for eldritch blast, at least)?

Answer 1

You can target the ground but the ground will be unharmed

Xanathar's Guide to Everything gives the following optional rule (p. 86)

If you cast a spell on someone or something that can’t be affected by the spell, nothing happens to that target, but if you used a spell slot to cast the spell, the slot is still expended. If the spell normally has no effect on a target that succeeds on a saving throw, the invalid target appears to have succeeded on its saving throw, even though it didn’t attempt one (giving no hint that the creature is in fact an invalid target). Otherwise, you perceive that the spell did nothing to the target.

This rule was designed for dealing with illusions, humanoid looking monstrosities and other circumstances where the player might believe the target was valid. It does suggest, however, that a character can choose to target an invalid target but the spell will not have any effect on the target.

This means that any excess attacks from a spell can be willfully directed at any invalid target without risking any damage to teammates or, if objects are invalid, property.

This answer goes into greater depth on the targeting of invalid targets