feat(flow.widgets): Add nil gui object (#6)

Skipped over by rendering library, and not included in output formspec.

README.md

@@ -271,6 +271,21 @@ gui.Label{label="I am centered!", align_h = "centre"},
 This applies to other elements as well, because using HBox and Spacer to centre
 elements creates unnecessary containers.
 
+#### `gui.Nil`
+
+A tool to allow for ternary-ish conditional widgets:
+
+```lua
+gui.VBox{
+    gui.Label{ label = "The box below is only present if the boolean is truthy" },
+    the_boolean and gui.Box{ color = "#FF0000" } or gui.Nil{},
+}
+```
+
+Use sparingly, flow still has to process each `Nil` object to be able to know to
+remove it, and thus could still slow things down. The fastest element is one
+that doesn't exist, and thus doesn't need processing.
+
 #### `gui.Stack`
 
 This container element places its children on top of each other. All child

init.lua

@@ -1173,6 +1173,14 @@ function gui.Spacer(def)
     return def
 end
 
+-- For use in inline <bool> and <a> or <b> type inline ifs
+function gui.Nil(def)
+    -- Tooltip elements are ignored when layouting and setting visible = false
+    -- ensures that the element won't get added to the resulting formspec
+    def.visible = false
+    return gui.Tooltip(def)
+end
+
 -- Prevent any further modifications to the gui table
 function gui_mt.__newindex()
     error("Cannot modifiy gui table")

test.lua

@@ -280,6 +280,19 @@ describe("Flow", function()
         ]])
     end)
 
+    it("ignores gui.Nil", function()
+        test_render(gui.VBox{
+            min_h = 5, -- Make sure gui.Nil doesn't expand
+            gui.Box{w = 1, h = 1, color = "red"},
+            gui.Nil{},
+            gui.Box{w = 1, h = 1, color = "green"},
+        }, [[
+            size[1.6,5.6]
+            box[0.3,0.3;1,1;red]
+            box[0.3,1.5;1,1;green]
+        ]])
+    end)
+
     it("registers inventory formspecs", function ()
         local stupid_simple_inv_expected =
             "formspec_version[6]" ..