Fix Lint/EmptyBlock warning

These blocks are intentionally empty.

Library/Homebrew/test/abstract_command_spec.rb

@@ -11,7 +11,9 @@ RSpec.describe Homebrew::AbstractCommand do
           switch "--foo"
           flag "--bar="
         end
-        define_method(:run) {}
+        define_method(:run) do
+          nil
+        end
       end
       stub_const("TestCat", test_cat)
     end
@@ -48,7 +50,9 @@ RSpec.describe Homebrew::AbstractCommand do
         before do
           tac = Class.new(described_class) do
             self.define_singleton_method(:command_name) { "t-a-c" }
-            define_method(:run) {}
+            define_method(:run) do
+              nil
+            end
           end
           stub_const("Tac", tac)
         end

Library/Homebrew/test/formula_validation_spec.rb

@@ -23,6 +23,7 @@ RSpec.describe Formula do
       expect do
         formula do
           define_method(:brew) do
+            nil
           end
         end
       end.to raise_error(RuntimeError, /\AThe method `brew` on #{described_class} was declared as final/)