Before the change `C.keys` returned keys captured in some previous test case that by chance captured `nil` value what made this test passed successfully. Now it returns keys captured in this test case.

For static analysis, it’s better to keep unused literal nodes.
If simply change `block_append` to fall through, both "unused literal ignored"
and "possibly useless use of a literal in void context" warnings
are shown for the same line. But it’s verbose then remove
"unused literal ignored" warning.

This kind of optimization is already implemented on compile.c.
`compile_block` calls `iseq_compile_each0` with `popped = 1` when NODE_BLOCK
has next.

ruby/test_default_gems.rb can define empty `DidYouMean` module
because of the following line (second require) in the
`lib/did_you_mean/did_you_mean.gemspec`:

```ruby
begin
  require_relative "lib/did_you_mean/version"
rescue LoadError # Fallback to load version file in ruby core repository
  require_relative "version"
end
```

It defines only `::DidYouMean::VERSION`.

However, in the `test/ruby/test_patten_matching.rb` assumes that
if `defined?(DidYouMean)` is true, then there is a method `DidYouMean.formatter=`
and this assumption fails all tests in `test/ruby/test_patten_matching.rb` if
there is only a `::DidYouMean::VERSION`.

To reproduce the failures, we need to repeat the following command:

`make test-all TESTS='-v ruby/test_default_gems.rb ruby/pattern_matching'`

(because the ruby/test_default_gems.rb should be run before the ruby/pattern_matching`)

This patch introduces more strict gurds.

It should also check for duplicate names.

[Bug #18990]

```
test/ruby/test_parse.rb:1384: warning: assigned but unused variable - obj
test/ruby/test_pattern_matching.rb:1162: warning: unused literal ignored
test/ruby/test_pattern_matching.rb:1165: warning: unused literal ignored
test/ruby/test_pattern_matching.rb:1161: warning: assigned but unused variable - a
test/ruby/test_pattern_matching.rb:1164: warning: assigned but unused variable - b
```

And a newline should be significant here.