**Current behavior:**
Creating an import file with enough blank rows to require import via batches will cause duplicate records from previous batches.

**Expected behavior:**
No duplicates.

**Steps to reproduce:**
*E.g., `-i contacts`*
1. Create a csv file with a partner record on row 1 (or 2 if using headers, just the name is enough for `res.partner`) and another record on row 2002 or higher

2. From contacts action button, select import records, upload the csv from step 1, import

3. See that the 2 records were imported 2x each

**Cause of the issue:**
When the `file_length` (literal number of lines) is big enough to require batches, but the actual number of rows with data is not, the upload mechanism will try to (in the case described here) upload 2 batches of records.

But because there isn't actually a full batch of records, the second batch will just be the same as the first (second batch starts from row 0 "bookmark", because all the data rows were processed in their entirety in the first batch).

**Fix:**
Make `file_length` returned by the server to client the length of the actual data rows parsed from the file, as opposed to the literal number of lines.

opw-4271105

Forward-Port-Of: odoo/odoo#185634

Consider the case that "Predict vendor bill product" is activated in
the settings and we upload / import a vendor bill with a single line.
In case the product on the line could not be determined by the import
function it is predicted afterwards (since the setting is enabled).

But changing the product triggers i.e. a recompute of the unit price
(`price_unit`) and taxes (`tax_ids`) based on the new product information.
This is not what we want since the aforementioned values should be
determined by the import even in case the product is not found.

After this commit we protect the unit price and taxes fields
during any (product) prediction in case they are set.
(We do update the unit price and tax if it is 0 or there are no taxes.)

Reproduce
1. Activate "Predict vendor bill product" in the settings
2. Create Customer OpsWay
   * country = Poland
   * VAT = PL8992922295
3. Create 3 Vendor Bills manually
   * date in the past
   * customer = OpsWay
   * single invoice line: with product "Commissions"
   * Confirm
4. Import the PDF attached on the task in Vendor Bills.
   * It has a single line with quantity 1 and unit price 1,322.40
5. A bill with a single line is created:
   product = "Commissions"
   unit price = 0.0
   total = 0

task-4279931

Forward-Port-Of: odoo/enterprise#73191
Forward-Port-Of: odoo/enterprise#72625