Make time-entry URI not required

[enriquezrene]

We need to make the uri field from time-entry optional.

[EliuX]

After we discussed this issue, we realized that the problem is not that the field uri is required, but that it is not nullable. So I will convert all strings fields into nullable ones.

[enriquezrene]

same with description @EliuX . I just tried again.