raise an exception instead of returning an empty list

as davidben points out in pyca#9926 we are calling a specific load
certificates function and an empty value doesn't necessarily mean empty
because PKCS7 contains multitudes. erroring is more correct.

src/cryptography/hazmat/backends/openssl/backend.py

@@ -1111,12 +1111,15 @@ def _load_pkcs7_certificates(self, p7) -> list[x509.Certificate]:
                 _Reasons.UNSUPPORTED_SERIALIZATION,
             )
 
-        certs: list[x509.Certificate] = []
         if p7.d.sign == self._ffi.NULL:
-            return certs
+            raise ValueError(
+                "The provided PKCS7 has no certificate data, but a cert "
+                "loading method was called."
+            )
 
         sk_x509 = p7.d.sign.cert
         num = self._lib.sk_X509_num(sk_x509)
+        certs: list[x509.Certificate] = []
         for i in range(num):
             x509 = self._lib.sk_X509_value(sk_x509, i)
             self.openssl_assert(x509 != self._ffi.NULL)

tests/hazmat/primitives/test_pkcs7.py

@@ -92,8 +92,8 @@ def test_load_pkcs7_unsupported_type(self, backend):
     def test_load_pkcs7_empty_certificates(self):
         der = b"\x30\x0B\x06\x09\x2A\x86\x48\x86\xF7\x0D\x01\x07\x02"
 
-        certificates = pkcs7.load_der_pkcs7_certificates(der)
-        assert certificates == []
+        with pytest.raises(ValueError):
+            pkcs7.load_der_pkcs7_certificates(der)
 
 
 # We have no public verification API and won't be adding one until we get