Fix incorrect parsing of requested Python version as empty version sp…

…ecifiers (#4289)

Before 0.2.10 we would parse `--python=python` as an executable name.
After #4214, we started treating
this as a Python version range request (with an empty version range).
This is not entirely unreasonable, but it was an unexpected regression
and I don't think `VersionRequest` should support empty ranges in its
`from_str` implementation without more consideration.

crates/uv-toolchain/src/discovery.rs

@@ -1237,6 +1237,9 @@ impl FromStr for VersionRequest {
             Ok(selector)
         // e.g. `>=3.12.1,<3.12`
         } else if let Ok(specifiers) = VersionSpecifiers::from_str(s) {
+            if specifiers.is_empty() {
+                return Err(Error::InvalidVersionRequest(s.to_string()));
+            }
             Ok(Self::Range(specifiers))
         } else {
             Err(Error::InvalidVersionRequest(s.to_string()))