ItemCreateJob: fix overwriting modification time with null datetime
Code after d5ab6167 which was porting to the new protocol no longer parsed a DATETIME argument. Instead for silent or no merges the mtime is passed via a FetchItemsResponse, so in that case the mtime is wrongly reset to null.
Not aware of any related bug, just seen by reading the code.
Edited by Friedrich W. H. Kossebau