openjdk: fix truststore-from-env patch for jdk10

storePropName will be jsseDefaultStore if the property isn't present, and
jsseDefaultStore is never null, so the branch to use the environment variable
would never be taken.

The env var is supposed to be preferred to jssecacerts, so we can use it as
the default in the call to System.getProperty, and use the null check to fall
back on jsseDefaultStore instead.