[FIX] base_import: import empty date/datetime

When a user imports a file with a datetime field set to a blank space,
the value is computed as 1900-01-01 00:00:00. However, when converted in
the user's TZ, this can lead to a value < 1900-01-01, which is not
recognized at the JS level as a valid value.

This makes the access to the record impossible, and possibly makes the
access to an app impossible. It is for example the case with Purchase,
where the order date is displayed on the default's action view (list
view).

We strip the value before testing if the value exists to avoid this
case.

opw-1826344

addons/base_import/models/base_import.py

@@ -622,9 +622,11 @@ class Import(models.TransientModel):
                     # versions, for both data and pattern
                     user_format = pycompat.to_native(options.get('%s_format' % field['type']))
                     for num, line in enumerate(data):
+                        if line[index]:
+                            line[index] = line[index].strip()
                         if line[index]:
                             try:
-                                line[index] = dt.strftime(dt.strptime(pycompat.to_native(line[index].strip()), user_format), server_format)
+                                line[index] = dt.strftime(dt.strptime(pycompat.to_native(line[index]), user_format), server_format)
                             except ValueError as e:
                                 raise ValueError(_("Column %s contains incorrect values. Error in line %d: %s") % (name, num + 1, e))
                             except Exception as e: