You will just have a little less flavor. I doubt it will make a huge deal. You could still boil up some concentrated DME in 1-2 cups of water and add that if you are really worried about it. But I would just let it ride.
BTW, I make all my recipes to 5.75 gallons in a 6.5 gal bucket. Although I do add enough fermentables to compensate. I lose between 0.5-1 gallons of beer in trub loss and don't want to get shorted the finished brew.
Primary #1: Big 50 Barleywine #2:Empty
Secondary #1: Peach Rhubarb Wine #2: Empty #3: Empty #4: Framboise Lambic
Kegged: Orange EPA, Piker Liker Porter
Bottles: Red Rye IPA, Dark Belgian Strong, Belgian Golden Strong, Kicked by a Moose Scotch Ale, Apfelwein, Russian Imperial Stout, Carmelite Tripel, Rhubarb Wine, Barolo Wine
On Deck: Umlaut my Kölsch V