[FIX] web: prevent access error in calendar

Prior to this commit:

- If the color attribute is set on a field on which the user might
  not have access to the model / record, a traceback is returned. However,
  in the case of a M2O, the name_get would be returned, so its is
  a pity that the color raise an access error.

After this commit:

- A search_read is made instead of a read, which returns nothing if
  the user does not have access to the model / record and does not raise
  a traceback.

closes odoo/odoo#90989

Signed-off-by: Géry Debongnie <ged@odoo.com>