gears.table.join: Ignore nil arguments (#2440)

When calling join with e.g. arguments (nil, {"a"}), then everything past
the nil was ignored, because the code internally used ipairs() to
iterate over the arguments and this stops at the first nil it
encounters.

Fix this by using select() to iterate over the arguments.

This also adds a unit test for this problem.

Signed-off-by: Uli Schlachter <psychon@znc.in>

lib/gears/table.lua

@@ -18,7 +18,8 @@ local gtable = {}
 -- @return A new table containing all keys from the arguments.
 function gtable.join(...)
     local ret = {}
-    for _, t in ipairs({...}) do
+    for i = 1, select("#", ...) do
+        local t = select(i, ...)
         if t then
             for k, v in pairs(t) do
                 if type(k) == "number" then

spec/gears/table_spec.lua

@@ -41,4 +41,11 @@ describe("gears.table", function()
             assert.is.equal(f(), nil)
         end)
     end)
+
+    describe("table.join", function()
+        it("nil argument", function()
+            local t = gtable.join({"a"}, nil, {"b"})
+            assert.is.same(t, {"a", "b"})
+        end)
+    end)
 end)