StoreAppend: fix NPE in get() with TX disabled
StoreAppend: fix NPE in get() with TX disabled

cocosli/mapdb,sumit784/mapdb,morfeo8marc/mapdb,jankotek/MapDB,pettermahlen/mapdb,SleimanJneidi/mapdb,binarytemple/mapdb,jankotek/mapdb,codeaudit/mapdb,jankotek/mapdb,hangum/mapdb,jankotek/MapDB,ffjiao/mapdb,lamfire/mapdb