Report unavailable cask platforms accurately

- `outdated_casks` treated a missing `version` as non-latest but then
  reported that the latest version was already installed.
- Handle the missing version explicitly and reuse the narrowed value so
  non-quiet output identifies the unavailable current platform.
- Strengthen the regression coverage for the warning path.

Library/Homebrew/cask/upgrade.rb

@@ -62,9 +62,13 @@ module Cask
             next false
           end
 
-          if cask.outdated?(greedy: true)
+          version = cask.version
+          if version.nil?
+            opoo "Not upgrading #{cask.token}, no version is available for the current platform" unless quiet
+            false
+          elsif cask.outdated?(greedy: true)
             true
-          elsif cask.version&.latest?
+          elsif version.latest?
             opoo "Not upgrading #{cask.token}, the downloaded artifact has not changed" unless quiet
             false
           else

Library/Homebrew/test/cask/upgrade_spec.rb

@@ -47,7 +47,7 @@ RSpec.describe Cask::Upgrade, :cask do
     allow(Homebrew::EnvConfig).to receive(:upgrade_auto_updates_casks?).and_return(true)
   end
 
-  it "excludes casks with no version for the current OS" do
+  it "warns and excludes casks with no version for the current platform" do
     cask = Homebrew::SimulateSystem.with(os: :linux) do
       Cask::Cask.new("macos-only") do
         on_macos do
@@ -56,7 +56,9 @@ RSpec.describe Cask::Upgrade, :cask do
       end
     end
 
-    expect(described_class.outdated_casks([cask], args:, force: true, quiet: true)).to be_empty
+    expect do
+      expect(described_class.outdated_casks([cask], args:, force: true, quiet: false)).to be_empty
+    end.to output(/Not upgrading macos-only, no version is available for the current platform/).to_stderr
   end
 
   context "when the upgrade is a dry run" do