Preserve t="str" on empty-string formula results

[ben-schreiber]

Fixes #356.

Problem

When the reader encounters a formula cell whose cached result is an empty string — written by Excel/LibreOffice as a self-closing <v/> with t="str" — the value is dropped and the cell decays to Empty, losing its string type. On write the t="str" attribute disappears, so =IF(cond,"","x") evaluating to "" becomes an untyped/blank cell. Non-empty string results are unaffected.

Root cause: in Cell::set_attributes, the Event::Start arm dispatches <v> on the declared t type, but the Event::Empty arm (which is how a self-closing <v/> arrives) only handled <f/> and ignored <v/> entirely.

Fix

Handle <v/> in the Event::Empty arm, dispatching on the declared type the same way the Event::Start arm does, so an empty string result keeps t="str".

Repro (before this PR)

input.xlsx cell: <c r="A1" t="str"><f>IF(TRUE,"","x")</f><v/></c>

let book = umya_spreadsheet::reader::xlsx::read(Path::new("input.xlsx")).unwrap();
let cell = book.get_sheet(&0).unwrap().get_cell((1, 1)).unwrap();
assert_eq!(cell.get_data_type(), "str"); // was "", now "str"

After the fix the written cell keeps its type: <c r="A1" t="str"><f>IF(TRUE,"","x")</f><v></v></c>.

Tests

Added empty_string_formula_result_keeps_str_type covering the read path. Full cargo test --lib passes (114 tests).

[MathNya]

@ben-schreiber
Thank you for the PR.
Since there were no issues, I’ll go ahead and merge it.
There’s an error in Clippy, but I’ll take care of that.