Add parameter validation to LdapSecurityRealm

Add parameter validation for the hashEncoding and hashCharset variables in the LdapSecurityRealm constructor.

Following what other security realms do, it should first verify the value is not null. If it's null, it should set these variables to sensible defaults:

• hashEncoding default value: Encoding.BASE64.
• hashCharset default value: StandardCharsets.UTF_8.