Enable empty STACK_OF(X509) for get0_signers()

Hi,
It's my first pull request on github, so please be kind...
I propose to enable a null value of STACK_OF(X509). I need it especially for the get0_signer() function in order to retrieve certificates associated with PKCS7 structure if you don't know them a priori.
It works for me, but I don't master the side effects of such change...
Any comments appreciated,
Best regards,
François