Tighten how input streams handle nothing, and related (#2847)

crates/nu-cli/tests/commands/math/sum.rs

@@ -33,13 +33,6 @@ fn all() {
     })
 }
 
-#[test]
-fn outputs_zero_with_no_input() {
-    let actual = nu!(cwd: ".", "math sum");
-
-    assert_eq!(actual.out, "0");
-}
-
 #[test]
 #[allow(clippy::unreadable_literal)]
 #[allow(clippy::float_cmp)]

crates/nu-cli/tests/commands/reduce.rs

@@ -103,5 +103,5 @@ fn error_reduce_empty() {
         )
     );
 
-    assert!(actual.err.contains("empty input"));
+    assert!(actual.err.contains("needs input"));
 }