Fix panic in calculator when pressing ± after AC (#5352)

* fix: handle empty string in calculator plus-minus button

calc_val() panics on empty string when AC is pressed before the
plus-minus button. Return 0.0 early for empty input.

Fixes #4858

* fix a few ci failures

* fix link check

---------

Co-authored-by: Evan Almloff <evanalmloff@gmail.com>

examples/01-app-demos/calculator.rs

@@ -148,6 +148,10 @@ fn app() -> Element {
 }
 
 fn calc_val(val: &str) -> f64 {
+    if val.is_empty() {
+        return 0.0;
+    }
+
     let mut temp = String::new();
     let mut operation = "+".to_string();