Respect empty initial value in 'append' and 'concat' actions

spec/actions_spec.lua

@@ -115,6 +115,21 @@ describe("actions", function()
       assert.same({foo = false, bar = true}, args)
    end)
 
+   it("'append' and 'concat' respect initial value", function()
+      local parser = Parser()
+      parser:option("-f"):count("*"):init(nil)
+      parser:option("-g"):args("*"):count("*"):action("concat"):init(nil)
+
+      local args = parser:parse{}
+      assert.same({}, args)
+
+      args = parser:parse{"-fabc", "-fdef", "-g"}
+      assert.same({f = {"abc", "def"}, g = {}}, args)
+
+      args = parser:parse{"-g", "abc", "def", "-g123", "-f123"}
+      assert.same({f = {"123"}, g = {"abc", "def", "123"}}, args)
+   end)
+
    it("for options allow setting initial stored value as non-string argument to default", function()
       local parser = Parser()
       parser:flag("--no-foo", "Foo the bar.", true):target("foo"):action("store_false")

src/argparse.lua

@@ -368,6 +368,7 @@ function actions.count(result, target, _, overwrite)
 end
 
 function actions.append(result, target, argument, overwrite)
+   result[target] = result[target] or {}
    table.insert(result[target], argument)
 
    if overwrite then
@@ -380,6 +381,8 @@ function actions.concat(result, target, arguments, overwrite)
       error("'concat' action can't handle too many invocations")
    end
 
+   result[target] = result[target] or {}
+
    for _, argument in ipairs(arguments) do
       table.insert(result[target], argument)
    end