455007 Fallback to default timestamp provider for shared but not
commited projects using jgit timestamp provider

When the commit is null, log an info message and return a timestamp
using the default provider. 
The commit can only be null if the project had been shared but nothing
had been commmited AND the <jgit.dirtyWorkingTree> parameter is set to
ignore.

Bug: 455007
Change-Id: If9b93553cbb4bcbd5d7f624006155a7ddd61645a
Signed-off-by: Martin Schreiber <m.schreiber@bachmann.info>