KSM-582: fix NPE use safe cast in KeeperRecordData.getField()

[maksimu]

Replace unsafe cast (as T) with safe cast operator (as? T) in getField() to prevent NPE when uploading files to records without FileRef field.

This fixes an issue where attempting to upload a file to a record without an initialized FileRef field would throw: "null cannot be cast to non-null type FileRef"

Fixes #698