Fix an error when running --only or --except options

This PR fixes an error when running `rubocop --only` or
`rubocop --except` options without cop name argument.

## Before

```consle
% rubocop --only
undefined method `empty?' for nil:NilClass
/Users/koic/.rbenv/versions/2.7.1/lib/ruby/gems/2.7.0/gems/rubocop-0.82.0/lib/rubocop/options.rb:87:in
`block in add_cop_selection_csv_option'
/Users/koic/.rbenv/versions/2.7.1/lib/ruby/gems/2.7.0/gems/rubocop-0.82.0/lib/rubocop/options.rb:206:in
`block in option'
/Users/koic/.rbenv/versions/2.7.1/lib/ruby/2.7.0/optparse.rb:1589:in
`block in parse_in_order'
/Users/koic/.rbenv/versions/2.7.1/lib/ruby/2.7.0/optparse.rb:1575:in `catch'
```

## After

```console
% rubocop --only
--only argument should be [COP1,COP2,...].
```

The same is true for `rubocop --except`.

CHANGELOG.md

@@ -12,6 +12,7 @@
 * [#7790](https://github.com/rubocop-hq/rubocop/issues/7790): Fix `--parallel` and `--ignore-parent-exclusion` combination. ([@jonas054][])
 * [#7881](https://github.com/rubocop-hq/rubocop/issues/7881): Fix `--parallel` and `--force-default-config` combination. ([@jonas054][])
 * [#7635](https://github.com/rubocop-hq/rubocop/issues/7635): Fix a false positive for `Style/MultilineWhenThen` when `then` required for a body of `when` is used. ([@koic][])
+* [#7905](https://github.com/rubocop-hq/rubocop/pull/7905): Fix an error when running `rubocop --only` or `rubocop --except` options without cop name argument. ([@koic][])
 
 ### Changes
 

lib/rubocop/options.rb

@@ -83,6 +83,12 @@ def add_only_options(opts)
 
     def add_cop_selection_csv_option(option, opts)
       option(opts, "--#{option} [COP1,COP2,...]") do |list|
+        unless list
+          message = "--#{option} argument should be [COP1,COP2,...]."
+
+          raise OptionArgumentError, message
+        end
+
         @options[:"#{option}"] =
           if list.empty?
             ['']

spec/rubocop/cli/cli_options_spec.rb

@@ -575,6 +575,15 @@ class SomeCop < Cop
           RESULT
       end
     end
+
+    context 'when a cop name is not specified' do
+      it 'displays how to use `--only` option' do
+        expect(cli.run(%w[--except -a Lint/NumberConverion])).to eq(2)
+        expect($stderr.string).to eq(<<~MESSAGE)
+          --except argument should be [COP1,COP2,...].
+        MESSAGE
+      end
+    end
   end
 
   describe '--except' do
@@ -694,6 +703,15 @@ class SomeCop < Cop
         end
       end
     end
+
+    context 'when a cop name is not specified' do
+      it 'displays how to use `--except` option' do
+        expect(cli.run(%w[--except])).to eq(2)
+        expect($stderr.string).to eq(<<~MESSAGE)
+          --except argument should be [COP1,COP2,...].
+        MESSAGE
+      end
+    end
   end
 
   describe '--lint' do