When `COMP_POINT` is invalid or out of range, return an empty list of
completions instead of raising `ValueError`. This matches behaviour for
`comp_point < 0 or comp_point > len(comp_line)` cases.
We currently return `@[]` in `splitCompletionLine` when there is
unexpected or unsupported input via environment variables, but let
exceptions during parsing propagate to the caller. Catching those
exceptions allows the caller to use same behaviour regardless of
the nature of unexpected input (either through env, or parsing).
Currently, callers don't seem to be aware of the exceptions, so
going with the behaviour used for environment errors of returning `@[]`.
