Use nan for empty fields in Calc formulas
* doc/org.texi (Formula syntax for Calc): Add explanation and example
for empty field.
* lisp/org-table.el (org-table-eval-formula): Use `keep-empty' in more
places.
(org-table-make-reference): Use nan (not a number) for empty fields in
Calc formulas.
* testing/lisp/test-org-table.el: Adapt expected for several
ert-deftest.
(test-org-table/empty-field): New examples dealing with empty fields.
(test-org-table/copy-field): New ert-deftest with examples dealing with
empty fields.
This makes it possible to write spreadsheet Calc formulas that check
for empty fields: To sum the first two columns unless one or both
empty:
$3 = if("$1" = "nan" || "$2" = "nan", string(""), $1 + $2); E