Ensure that the listidentifier postprocessor does not change the value if no match can be made

It's a leftover of BATCHIMP-26 that didn't make it to 2.3. See the comment on BATCHIMP-26:

In the same idea of making sure that if no reverse mapping is possible then the values from the file are not messed up we need to make sure that the value read from the file (https://github.com/xwiki-contrib/batchimport/blob/master/xwiki-batchimport-postprocessors/dblistidentifier/src/main/java/org/xwiki/batchimport/postprocessors/DBListIdentifierPostprocessor.java#L103 ) is identical to the value prepared for marshalling in the objects on line (https://github.com/xwiki-contrib/batchimport/blob/master/xwiki-batchimport-postprocessors/dblistidentifier/src/main/java/org/xwiki/batchimport/postprocessors/DBListIdentifierPostprocessor.java#L121 ), in terms of whitespaces that may be trimmed upon splitting and re-joining. See also BATCHIMP-30 that refers to a manipulation that is done on the value even when not needed.