Avoid error message when destroying key w/o secdiscardable file

Since commit 08f4bdfe ("Don't use a secdiscardable file for keys
encrypted by SP") (https://r.android.com/2242561), some keys don't use a
secdiscardable file.  Currently if such a key is destroyed, an ERROR
message like the following is logged:

    E secdiscard: Secure discard open failed for: /data/misc/vold/user_keys/ce/14/current/secdiscardable

This case is expected, so it should not be an ERROR.  Fix this by only
passing the secdiscardable file to the secdiscard program if it exists.

Bug: 232452368
Change-Id: I490289dfdaf0db6c3f4fb507509095e0033e2f69