Fix SEGV on null StringIO #417

Job	Run time
ruby-versions / Generate Ruby versions	4s
windows-latest 3.0	50s
windows-latest 3.1	51s
windows-latest 3.3	1m 3s
windows-latest 3.4	44s
windows-latest 3.2	1m 6s
windows-latest jruby-head	1m 24s
windows-latest mingw	49s
windows-latest mswin	1m 3s
windows-latest 2.7	1m 18s
	9m 12s

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