Make uplevel suitable as the option to Kernel#warn

Make Gem::BUNDLED_GEMS.uplevel returns `nil` if `require` frame is not
found, for the simplicity.

Author: nobu

lib/bundled_gems.rb

@@ -49,12 +49,7 @@ def self.replace_require(specs)
       kernel_class.send(:alias_method, :no_warning_require, :require)
       kernel_class.send(:define_method, :require) do |name|
         if message = ::Gem::BUNDLED_GEMS.warning?(name, specs: spec_names)
-          uplevel = ::Gem::BUNDLED_GEMS.uplevel
-          if uplevel > 0
-            Kernel.warn message, uplevel: uplevel
-          else
-            Kernel.warn message
-          end
+          Kernel.warn message, uplevel: ::Gem::BUNDLED_GEMS.uplevel
         end
         kernel_class.send(:no_warning_require, name)
       end
@@ -90,7 +85,7 @@ def self.uplevel
         break
       end
     end
-    require_found ? 1 : frame_count - 1
+    require_found ? 1 : frame_count > 1 ? frame_count - 1 : nil
   end
 
   def self.warning?(name, specs: nil)