fix: fix new rubocop offense Style/EmptyStringInsideInterpolation

Author: jcouball

lib/process_executer/options/base.rb

@@ -268,7 +268,7 @@ def assert_no_unknown_options
 
         raise(
           ArgumentError,
-          "Unknown option#{unknown_options.count > 1 ? 's' : ''}: #{unknown_options.join(', ')}"
+          "Unknown option#{'s' if unknown_options.count > 1}: #{unknown_options.join(', ')}"
         )
       end
     end

lib/process_executer/result.rb

@@ -102,7 +102,7 @@ def success?
     # @return [String]
     #
     def to_s
-      "#{super}#{timed_out? ? " timed out after #{options.timeout_after}s" : ''}"
+      "#{super}#{" timed out after #{options.timeout_after}s" if timed_out?}"
     end
   end
 end