Can you play Salvage without target?

Yes, you can play Salvage without a target.

According to the current text of the card (which received errata), it reads: "You may kill up to one gear. Draw 1."

Under Rule 352.13:

"If a card specifies that a player chooses “any number” or “up to” some number of Game Objects to be affected, they may choose any number of available targets, including zero. If they choose zero, the spell or ability can be played without any targets."

Because the card uses the "up to one" language, choosing zero gears is a valid choice during the targeting step. This allows you to play the spell solely for the "Draw 1" effect even if there are no gears on the board.

Historical Note: Older versions of Salvage were templated as "You may kill a gear. Draw 1." Under that old wording, you were required to have a valid gear target to put the spell on the chain (even if you chose not to kill it upon resolution). The errata to "up to one" was specifically made to ensure the card is not "dead" in hand when no gears are in play.