Grover algorithm when element not present in the array/database
The most basic version of Grover's algorithm has a fixed number of iterations and assumes there is one marked item. If there is no marked item instead of one marked item, you will do those iterations but then get a random answer.

As mentioned in the wikipedia article on Grover's algorithm, the algorithm can be extended to work when there are an unknown number of marked items while still running in $O(\sqrt{N})$ time. There's just more details to work out.