fix(defaultAction) fallback to default action on emtpy expression

[asmodeus812]

This should allow the expression function to return nil, to by default execute the default action of the list, as there are no easy ways to extract the name of the default action for the current list. doAction by default if no 'name' is provided would fallback to the defaultAction.

[asmodeus812]

The reason is to be able to do something like that

function _G.list_default_action(ctx)
        if ctx and #ctx.targets > 1 then
            -- send to qflist
            return "quickfix"
        else
            -- default
            return nil
        end
    end

        function! Listaction(ctx)
            " do execute quickfix or default action
            return v:lua.list_default_action(a:ctx)
        endfunction

   "<CR>": "expr:Listaction",

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.56%. Comparing base (846994b) to head (30f56c7).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4954      +/-   ##
==========================================
+ Coverage   98.54%   98.56%   +0.01%     
==========================================
  Files         273      273              
  Lines       26060    26060              
  Branches     5386     5385       -1     
==========================================
+ Hits        25682    25687       +5     
+ Misses        223      219       -4     
+ Partials      155      154       -1     

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