Allow overwriting a list with an empty list using (=) operatorsam/overwrite-list-with-empty-list

I found myself writing the following split rules for an element:

    public:
      bst:
        split-rules:
          runtime:
            (=): []

          devel:
            (>):
            - |
              %{bindir}/*

The aim was to put all of this element's binaries into the 'devel'
domain, and make sure nothing went into the 'runtime' domain by
replacing the built-in rules for that domain with an empty list.

That wasn't working though because BuildStream was using [] (empty list)
as a sentinel to mean "do nothing", which is fine for the prepend and
append operators (prepending an empty list to a list has no effect) but
is not really correct for the overwrite operator.

This commit fixes that issue and adds a test.

0 files changed, 0 insertions, 0 deletions