Make 'for' implicitly filter out nothings (#4536)

* Make 'for' implicitly filter out nothings

* Fix test

crates/nu-command/src/core_commands/for_.rs

@@ -109,6 +109,7 @@ impl Command for For {
                         Err(error) => Value::Error { error },
                     }
                 })
+                .filter(|x| !x.is_nothing())
                 .into_pipeline_data(ctrlc)),
             Value::Range { val, .. } => Ok(val
                 .into_range_iter()?
@@ -146,6 +147,7 @@ impl Command for For {
                         Err(error) => Value::Error { error },
                     }
                 })
+                .filter(|x| !x.is_nothing())
                 .into_pipeline_data(ctrlc)),
             x => {
                 stack.add_var(var_id, x);

crates/nu-protocol/src/value/mod.rs

@@ -562,6 +562,10 @@ impl Value {
         }
     }
 
+    pub fn is_nothing(&self) -> bool {
+        matches!(self, Value::Nothing { .. })
+    }
+
     /// Create a new `Nothing` value
     pub fn nothing(span: Span) -> Value {
         Value::Nothing { span }

src/tests/test_engine.rs

@@ -149,7 +149,7 @@ fn proper_variable_captures_with_nesting() -> TestResult {
 
 #[test]
 fn proper_variable_for() -> TestResult {
-    run_test(r#"for x in 1..3 { if $x == 2 { "bob" } } | get 1"#, "bob")
+    run_test(r#"for x in 1..3 { if $x == 2 { "bob" } } | get 0"#, "bob")
 }
 
 #[test]