Add MixUnused.Analyzer test for app: nil branch

test/mix_unused/analyzer_test.exs

@@ -104,6 +104,21 @@ defmodule MixUnused.AnalyzerTest do
       assert result == []
     end
 
+    test ":app=nil hits in_app?(_, nil) → true (no filtering)" do
+      # When app is nil, in_app? returns true for every module — so the
+      # analyzer scans everything in the compile path, not just the project.
+      # This exercises line 86 (defp in_app?(_module, nil), do: true).
+      tmp = Path.join(System.tmp_dir!(), "mu_app_nil_#{System.unique_integer([:positive])}")
+      File.mkdir_p!(tmp)
+
+      try do
+        result = Analyzer.analyze(app: nil, compile_path: tmp)
+        assert result == []
+      after
+        File.rm_rf(tmp)
+      end
+    end
+
     test "honors :compile_path option pointing to an empty directory" do
       tmp = Path.join(System.tmp_dir!(), "mu_empty_#{System.unique_integer([:positive])}")
       File.mkdir_p!(tmp)