Fix specs for Ruby 3.2

Since `Object#=~` has been removed in Ruby 3.2, the following code
raises NoMethodError when `asset` provided as `Hash`.
https://github.com/seanpdoyle/ember-cli-rails-assets/blob/v0.7.0/lib/ember_cli/assets/asset_map.rb#L36

Ref: https://bugs.ruby-lang.org/issues/15231

This spec should be updated with empty value as `nil` due to type consistency.

spec/lib/ember_cli/assets/asset_map_spec.rb

@@ -5,7 +5,7 @@
     it "includes the most recent javascript build artifacts" do
       asset_map = {
         "assets" => {
-          "not-a-match" => {},
+          "not-a-match" => nil,
           "bar.js" => "bar-abc123.js",
           "vendor.js" => "vendor-abc123.js",
         },
@@ -42,7 +42,7 @@
     it "includes the most recent stylesheet build artifacts" do
       asset_map = {
         "assets" => {
-          "not-a-match" => {},
+          "not-a-match" => nil,
           "bar.css" => "bar-abc123.css",
           "vendor.css" => "vendor-abc123.css",
         },