Fix panic on empty-string value for list options

[nikolauspschuetz]

What

getListValues (go/libraries/utils/argparser/parser.go) checked arg[0] == '-' without a length guard, so an empty-string value for any list-valued option panicked:

panic: runtime error: index out of range [0] with length 0
    .../argparser/parser.go:499

This is reachable from the CLI for any option built with SupportsStringList (i.e. AllowMultipleOptions), e.g.:

dolt log --not ""
dolt log -t ""
dolt diff --include-cols ""

Each aborts with a runtime panic instead of accepting the empty value.

Fix

Test for the empty string before indexing arg[0]:

if arg == "--" || (arg != "" && arg[0] == '-') {

An empty value is now collected like any other non-flag argument, and a following option (-l "" -f) still terminates the list.

Testing

Added TestArgParserListEmptyValue, which panics before this change and passes after. The full argparser package tests, go vet, and gofmt are clean.