Handle empty signed data in PKCS7 #885

Job	Run time
openssl-1.0.2u	3m 37s
openssl-1.1.0l	2m 40s
openssl-1.1.1w	2m 32s
ruby-versions / Generate Ruby versions	8s
openssl-3.0.12	5m 28s
openssl-3.1.4	5m 11s
libressl-3.1.5	4m 8s
libressl-3.2.7	3m 51s
libressl-3.3.6	3m 32s
libressl-3.4.3	3m 42s
libressl-3.5.3	4m 11s
libressl-3.6.3	3m 50s
libressl-3.7.3	3m 16s
libressl-3.8.1	3m 5s
openssl-3.0.12 fips	4m 30s
openssl-3.1.4 fips	4m 13s
openssl-head	5m 22s
openssl-head fips	5m 16s
ubuntu-22.04 2.7	1m 32s
ubuntu-22.04 3.0	2m 37s
ubuntu-22.04 3.1	1m 34s
ubuntu-22.04 3.2	1m 38s
ubuntu-22.04 head	1m 21s
ubuntu-22.04 truffleruby	2m 14s
ubuntu-22.04 truffleruby-head	2m 30s
ubuntu-20.04 2.7	1m 24s
ubuntu-20.04 3.0	1m 15s
ubuntu-20.04 3.1	1m 9s
ubuntu-20.04 3.2	1m 10s
ubuntu-20.04 head	1m 18s
ubuntu-20.04 truffleruby-head	1m 49s
macos-latest 2.7	1m 54s
macos-latest 3.0	1m 12s
macos-latest 3.1	1m 27s
macos-latest 3.2	1m 32s
macos-latest head	1m 22s
macos-latest truffleruby-head	4m 7s
windows-latest 2.7	6m 21s
windows-latest 3.0	6m 7s
windows-latest 3.1	5m 33s
windows-latest 3.2	7m 45s
windows-latest ucrt	5m 50s
windows-latest mswin	3m 28s
	2h 16m 41s

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