fix: guard against empty sources vector

[OnFaber]

Hi sometimes the function consult--multi-lookup give me:

cond: Args out of range: [], 0

because it expect at least on source.

[armindarvish]

@OnFaber Can you give me examples of when and why this happens? Naturally, it should at least have one source, so how do you end up with no sources?

[OnFaber]

Hi! I can’t tell you how I ended up with no sources, I just installed it and ran M-x.
It’s been some time since it happened :/