[FIX] hr_timesheet: changing project resets the employee
[FIX] hr_timesheet: changing project resets the employee

The preprocess method checked if the project_id value
was given to determine if it was a timesheet or a normal
analytic line (since a timesheet is an AAL with project_id).
The problem came in the write, when changing only the project
value, the preprocess use the timesheet case, but since the
employee field was not changed, no value was passed, and it
compute the default one. So changing the project of an existing
timesheet reset the employee to the current one.

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