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

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

packages/rsx/src/partial_closure.rs

@@ -11,7 +11,7 @@ use syn::{BoundLifetimes, ExprClosure};
 
 /// A closure whose body might not be valid rust code but we want to interpret it regardless.
 /// This lets us provide expansions in way more cases than normal closures at the expense of an
-/// increased mainteance burden and complexity.
+/// increased maintenance burden and complexity.
 ///
 /// We do our best to reuse the same logic from partial exprs for the body of the PartialClosure.
 /// The code here is simply stolen from `syn::ExprClosure` and lightly modified to work with