KsmRecordService.getField() may throw a NullPointerException if getCustom() returns null

The getCustom() function of KeeperRecordData as defined by the KSM SDK is permitted to return null (it is annotated @Nullable), however Guacamole's support for KSM assumes that it will always return a List, resulting in a NullPointerException if it ever does return null:

java.lang.NullPointerException
        at org.apache.guacamole.vault.ksm.secret.KsmRecordService.getField(KsmRecordService.java:241)
        at org.apache.guacamole.vault.ksm.secret.KsmRecordService.getField(KsmRecordService.java:312)
        at org.apache.guacamole.vault.ksm.secret.KsmRecordService.getHostname(KsmRecordService.java:397)
...

Things should be corrected as necessary to handle null identically to an empty List.