Check for empty string, before appending additional detail.  Inexplicably this is causing a test
failure for user Laurent V:

testPrepareFailureRollbackFailure(bitronix.tm.twopc.Phase1FailureTest): null
expected:<...Exception(XAER_RMERR[]) - resource 2 prepa...>
but was:  <...Exception(XAER_RMERR[ - ]) - resource 2 prepa...>