Fix TestPatternMatching#test_deconstruct_keys test

[andrykonchin]

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

[andrykonchin]

Is there anything I can do to have CI passed (a single failure seems irrelevant to the change) and this PR merged?

The failure is:

[721/885] 3312=test_sleep
  NMAKE : fatal error U1058: terminated by user
  Stop.
  Terminate batch job (Y/N)? 
  #<Thread:0x00000131edb21798 D:/a/ruby/ruby/src/test/ruby/test_io.rb:890 run> terminated with exception (report_on_exception is true):
  [722/885] 3312=test_name_error
  D:/a/ruby/ruby/src/test/ruby/test_io.rb:890:in 'IO#read': stream closed in another thread (IOError)
  [723/885] 5[816](https://github.com/ruby/ruby/actions/runs/9030242216/job/24814120038?pr=10757#step:24:817)=json_string_matching_test
  	from D:/a/ruby/ruby/src/test/ruby/test_io.rb:890:in 'block (3 levels) in TestIO#test_copy_stream_socket3'
  [724/885] 5732=test_safe_load

[k-tsj]

@andrykonchin
It seems failed spec is not related to the change, so I merged your PR. Thank you!