Fix dynamic completion audit crash

Library/Homebrew/rubocops/lines.rb

@@ -662,7 +662,10 @@ def audit_formula(formula_nodes)
           methods.each do |method|
             next unless method.source.include?("shells:")
 
-            shells << method.source.match(/shells: \[(:bash|:zsh|:fish)\]/).captures.first
+            shell = method.source[/shells: \[(:bash|:zsh|:fish)\]/, 1]
+            next if shell.nil?
+
+            shells << shell
             offenses << method
           end
 

Library/Homebrew/test/rubocops/lines/generate_completions_spec.rb

@@ -143,5 +143,23 @@ def install
         end
       RUBY
     end
+
+    it "does not report an offense when shells are generated dynamically" do
+      expect_no_offenses(<<~RUBY)
+        class Foo < Formula
+          name "foo"
+
+          def install
+            generate_completions_from_executable(bin/"foo", "completions")
+            [:zsh, :bash].each do |shell|
+              generate_completions_from_executable(
+                bin/"foo", "completions", shell.to_s, "bar", shells: [shell], base_name: "bar",
+                shell_parameter_format: :none
+              )
+            end
+          end
+        end
+      RUBY
+    end
   end
 end