fix: avoid empty autocomplete input panic

[rupayon123]

Description

Autocomplete on an empty prompt currently panics because SuggestCompletion indexes currentInput[len(currentInput)-1] before checking its length. Return no suggestions for empty input, which lets the shell remain at the prompt. A regression test reproduces the index-out-of-range panic before the change and passes afterward.

Validation: go test -p 1 ./... -count=1, go vet -p 1 ./..., go build -p 1 ./..., gofmt -l on changed files, and git diff --check pass locally with Go 1.27.1. The repository's Go 1.22.1 binary cannot run tests on this macOS host (dyld: missing LC_UUID load command), so CI should verify the pinned toolchain. The local tests did not exercise an external sqld server.

Related Issues

• Closes Autocomplete on empty input panics #159

Visual reference

Not applicable to this CLI crash; the before/after evidence is the regression test.

AI assistance was used to investigate, implement, and test this change; I reviewed the diff and validation results.