port empty command (#528)

* port empty command

* Pull upstream and use test_data() function for example tests

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

@@ -463,6 +463,21 @@ impl Value {
         }
     }
 
+    /// Check if the content is empty
+    pub fn is_empty(self) -> bool {
+        match self {
+            Value::String { val, .. } => val.is_empty(),
+            Value::List { vals, .. } => {
+                vals.is_empty() && vals.iter().all(|v| v.clone().is_empty())
+            }
+            Value::Record { cols, vals, .. } => {
+                cols.iter().all(|v| v.is_empty()) && vals.iter().all(|v| v.clone().is_empty())
+            }
+            Value::Nothing { .. } => true,
+            _ => false,
+        }
+    }
+
     /// Create a new `Nothing` value
     pub fn nothing(span: Span) -> Value {
         Value::Nothing { span }