Fix cryptfs RSA signing with keystore2

Fix KeymasterOperation::updateCompletely() to not treat an empty output
as an error, since for RSA signing (used by cryptfs / FDE) it is
expected that the output from update() be empty.  The output is instead
produced at the end by finish().

This is one of a set of changes that is needed to get FDE working again
so that devices that launched with FDE can be upgraded to Android 12.

Bug: 186165644
Change-Id: Icf120f8b9526d051d0ebe16bc8ad1edf712241e1
diff --git a/Keymaster.cpp b/Keymaster.cpp
index bb26b64..08e06d1 100644
--- a/Keymaster.cpp
+++ b/Keymaster.cpp
@@ -81,14 +81,7 @@
             ks2Operation = nullptr;
             return false;
         }
-
-        if (!output) {
-            LOG(ERROR) << "Keystore2 operation update didn't return output.";
-            ks2Operation = nullptr;
-            return false;
-        }
-
-        consumer((const char*)output->data(), output->size());
+        if (output) consumer((const char*)output->data(), output->size());
     }
     return true;
 }