Add specs for anonymous and null object doubles docs

spec/docs/null_objects_spec.cr

@@ -0,0 +1,16 @@
+require "../spec_helper"
+
+# https://gitlab.com/arctic-fox/spectator/-/wikis/Null-Objects
+Spectator.describe "Null Objects Docs" do
+  double :my_double
+
+  it "returns itself for undefined methods" do
+    dbl = double(:my_double).as_null_object
+    expect(dbl.some_undefined_method).to be(dbl)
+  end
+
+  it "can be used to chain methods" do
+    dbl = double(:my_double).as_null_object
+    expect(dbl.foo.bar.baz).to be(dbl)
+  end
+end