Add parameter validation in the constructor for KeyStoreBackedSecurityRealm to ensure the keystore is not null.

An example of how to add a check to make sure a parameter is not null can be found here:
https://github.com/wildfly-security/wildfly-elytron/blob/0bc21f17dab0507f7a5cdf7d2e7ea14d44eae12a/credential/base/src/main/java/org/wildfly/security/credential/PasswordCredential.java#L51