BUG: manage raw ods file without cell cache

[guillett]

• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

import ezodf
ods = ezodf.newdoc("ods", "test_unempty_cells.ods")
sheet = ezodf.Sheet("base", size=(6, 1))
ods.sheets += sheet

sheet[0, 0].set_value("Column 1")
for i in range(1, 6, 2):
    sheet[i, 0].set_value(i)
    #sheet[i, 0].append(ezodf.Paragraph(str(i)))
ods.save()

should generate

	Column 1
0	1
1	nan
2	3
3	nan
4	5

but it generates (an empty DataFrame)

Column 1

A hacky fix in building the ODS file is commented out in the snippet sheet[i, 0].append(ezodf.Paragraph(str(i))).
There is a cache in ODS file and data is correctly read from the origin but the logic to determine if a full row is empty mistakenly relies on the cache.

This PR removes that logic et fixes the bug.

[mroeschke]

Nice thanks @guillett