Allow syncing to empty virtual environment directories (#9427)

As discussed in https://github.com/astral-sh/uv/issues/9423, it's
confusing that we do not allow `uv sync` just because the `.venv`
directory _exists_. This change matches `uv venv`.

crates/uv-python/src/environment.rs

@@ -43,6 +43,7 @@ pub struct InvalidEnvironment {
 #[derive(Debug, Clone)]
 pub enum InvalidEnvironmentKind {
     NotDirectory,
+    Empty,
     MissingExecutable(PathBuf),
 }
 
@@ -128,6 +129,7 @@ impl std::fmt::Display for InvalidEnvironmentKind {
             Self::MissingExecutable(path) => {
                 write!(f, "missing Python executable at `{}`", path.user_display())
             }
+            Self::Empty => write!(f, "directory is empty"),
         }
     }
 }
@@ -178,6 +180,14 @@ impl PythonEnvironment {
             .into());
         }
 
+        if venv.read_dir().is_ok_and(|mut dir| dir.next().is_none()) {
+            return Err(InvalidEnvironment {
+                path: venv,
+                kind: InvalidEnvironmentKind::Empty,
+            }
+            .into());
+        }
+
         let executable = virtualenv_python_executable(&venv);
 
         // Check if the executable exists before querying so we can provide a more specific error