org-table.el: Fix range len bugs and inconsistencies
* lisp/org-table.el (org-table-eval-formula): Keep empty fields during
preprocessing.
(org-table-make-reference): A range with only empty fields should not
always return 0 but also empty string, consistent with field reference
of an empty field. Use future design for nan but replicate current
behavior.
* testing/lisp/test-org-table.el: Adapt expected for several
ert-deftest.
The range len bugs may lead to wrong calculations for range references
with empty fields when the range len is relevant. Affects typically
Calc vmean on simple range and without format specifier EN. Also
Lisp with e. g. `length' on simple range or with L.