Workaround for Style/RedundantBegin when using JRuby 9.2

Workaround for jruby/jruby#6642.

This commits prevent the following error when using JRuby 9.2.

```console
% ruby -v
jruby 9.2.17.0 (2.5.8) 2021-03-29 84d363da97 Java HotSpot(TM) 64-Bit
Server VM 25.271-b09 on 1.8.0_271-b09 +jit [darwin-x86_64]

% bundle exec rake documentation_syntax_check
[warn]: Syntax error in
`lib/rubocop/cop/lint/shadowed_exception.rb`:(118,18): syntax error,
unexpected keyword_rescue

[warn]: ParserSyntaxError: syntax error in
`lib/rubocop/cop/lint/shadowed_exception.rb`:(118,18): syntax error,
unexpected keyword_rescue

[warn]: Stack trace:
        /Users/koic/.rbenv/versions/jruby-9.2.17.0/lib/ruby/gems/shared/gems/yard-0.9.26/lib/yard/parser/ruby/ruby_parser.rb:601:in
        `on_parse_error'
        org/jruby/ext/ripper/RubyRipper.java:347:in `parse'
        /Users/koic/.rbenv/versions/jruby-9.2.17.0/lib/ruby/gems/shared/gems/yard-0.9.26/lib/yard/parser/ruby/ruby_parser.rb:56:in
        `parse'
        /Users/koic/.rbenv/versions/jruby-9.2.17.0/lib/ruby/gems/shared/gems/yard-0.9.26/lib/yard/parser/ruby/ruby_parser.rb:17:in
        `parse'
        /Users/koic/.rbenv/versions/jruby-9.2.17.0/lib/ruby/gems/shared/gems/yard-0.9.26/lib/yard/parser/source_parser.rb:442:in
        `parse'
        /Users/koic/.rbenv/versions/jruby-9.2.17.0/lib/ruby/gems/shared/gems/yard-0.9.26/lib/yard/parser/source_parser.rb:46:in
        `block in parse'

Files:         569
Modules:        88 (   13 undocumented)
Classes:       531 (    2 undocumented)
Constants:     892 (  879 undocumented)
Attributes:     31 (    0 undocumented)
Methods:      1186 ( 1060 undocumented)
 28.37% documented
rake aborted!
NoMethodError: undefined method `each' for nil:NilClass
/Users/koic/src/github.com/rubocop/rubocop/Rakefile:105:in `block in
/Users/koic/src/github.com/rubocop/rubocop/Rakefile'
/Users/koic/src/github.com/rubocop/rubocop/lib/rubocop/cop/registry.rb:207:in
`each'
/Users/koic/src/github.com/rubocop/rubocop/Rakefile:96:in `block in
<main>'
/Users/koic/.rbenv/versions/jruby-9.2.17.0/bin/bundle:23:in `<main>'
Tasks: TOP => documentation_syntax_check
(See full trace by running task with --trace)
```

lib/rubocop/cop/lint/shadowed_exception.rb

@@ -111,12 +111,18 @@ def evaluate_exceptions(group)
 
           if rescued_exceptions.any?
             rescued_exceptions.each_with_object([]) do |exception, converted|
-              RuboCop::Util.silence_warnings do
-                # Avoid printing deprecation warnings about constants
-                converted << Kernel.const_get(exception.source)
+              # FIXME: Workaround `rubocop:disable` comment for JRuby.
+              #        https://github.com/jruby/jruby/issues/6642
+              # rubocop:disable Style/RedundantBegin
+              begin
+                RuboCop::Util.silence_warnings do
+                  # Avoid printing deprecation warnings about constants
+                  converted << Kernel.const_get(exception.source)
+                end
+              rescue NameError
+                converted << nil
               end
-            rescue NameError
-              converted << nil
+              # rubocop:enable Style/RedundantBegin
             end
           else
             # treat an empty `rescue` as `rescue StandardError`