fix/expr: EXIT_CODE=1 for any return output parsable to 0 (or empty)

src/uu/expr/src/expr.rs

@@ -74,7 +74,7 @@ fn process_expr(token_strings: &[&str]) -> Result<String, String> {
 
 fn print_expr_ok(expr_result: &str) -> UResult<()> {
     println!("{}", expr_result);
-    if expr_result == "0" || expr_result.is_empty() {
+    if expr_result.parse::<i32>() == Ok(0) || expr_result.is_empty() {
         Err(1.into())
     } else {
         Ok(())