linenoise.cpp: allow completions from empty input
linenoise.cpp: allow completions from empty input

Fixes #1742.

vcunat/nix,peti/nix,NixOS/nix,dtzWill/nix,layus/nix,dtzWill/nix,pikajude/nix,pikajude/nix,dtzWill/nix,zimbatm/nix,vcunat/nix,shlevy/nix,rycee/nix,dtzWill/nix,shlevy/nix,shlevy/nix,dezgeg/nix,dezgeg/nix,vcunat/nix,rycee/nix,dezgeg/nix,rycee/nix,dtzWill/nix,ehmry/nix,zimbatm/nix,ehmry/nix,layus/nix,dezgeg/nix,ehmry/nix,NixOS/nix,dezgeg/nix,layus/nix,layus/nix,peti/nix,shlevy/nix,NixOS/nix,rycee/nix,rycee/nix,layus/nix,ehmry/nix,peti/nix,peti/nix,ehmry/nix,NixOS/nix,NixOS/nix,peti/nix,ehmry/nix,vcunat/nix,zimbatm/nix,pikajude/nix,shlevy/nix,zimbatm/nix