Handle empty signed data in PKCS7 #892

Job	Run time
openssl-1.0.2u	2m 38s
openssl-1.1.0l	1m 32s
openssl-1.1.1w	1m 46s
openssl-3.0.12	2m 49s
openssl-3.1.4	2m 55s
libressl-3.1.5	2m 18s
libressl-3.2.7	3m 20s
libressl-3.3.6	3m 11s
libressl-3.4.3	2m 45s
libressl-3.5.3	2m 17s
libressl-3.6.3	2m 22s
libressl-3.7.3	2m 20s
libressl-3.8.1	2m 9s
openssl-3.0.12 fips	4m 54s
openssl-3.1.4 fips	5m 38s
openssl-head	3m 38s
openssl-head fips	5m 54s
ruby-versions / Generate Ruby versions	3s
ubuntu-22.04 2.7	58s
ubuntu-22.04 3.0	52s
ubuntu-22.04 3.1	1m 14s
ubuntu-22.04 3.2	55s
ubuntu-22.04 head	1m 12s
ubuntu-22.04 truffleruby	1m 34s
ubuntu-22.04 truffleruby-head	1m 43s
ubuntu-20.04 2.7	1m 12s
ubuntu-20.04 3.0	1m 13s
ubuntu-20.04 3.1	1m 11s
ubuntu-20.04 3.2	1m 2s
ubuntu-20.04 head	1m 15s
ubuntu-20.04 truffleruby-head	1m 37s
macos-latest 2.7	2m 1s
macos-latest 3.0	1m 0s
macos-latest 3.1	1m 5s
macos-latest 3.2	1m 20s
macos-latest head	58s
macos-latest truffleruby-head	3m 57s
windows-latest 2.7	5m 46s
windows-latest 3.0	5m 48s
windows-latest 3.1	6m 40s
windows-latest 3.2	4m 9s
windows-latest ucrt	5m 57s
windows-latest mswin	3m 47s
	1h 50m 55s

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