use command: Don't create a variable with empty record if it doesn't define any constants (#14051)

# Description
Fixes: #13967

The key changes lays in `nu-protocol/src/module.rs`, when resolving
import pattern, nushell only needs to bring `$module` with a record
value if it defines any constants.

# User-Facing Changes
```nushell
module spam {}
use spam
```
Will no longer create a `$spam` variable with an empty record.

# Tests + Formatting
Adjusted some tests and added some tests.

crates/nu-protocol/src/module.rs

@@ -161,20 +161,25 @@ impl Module {
             }
 
             let span = self.span.unwrap_or(backup_span);
-            let const_record = Value::record(
-                const_rows
-                    .into_iter()
-                    .map(|(name, val)| (String::from_utf8_lossy(&name).to_string(), val))
-                    .collect(),
-                span,
-            );
+
+            // only needs to bring `$module` with a record value if it defines any constants.
+            let constants = if const_rows.is_empty() {
+                vec![]
+            } else {
+                vec![(
+                    final_name.clone(),
+                    Value::record(
+                        const_rows
+                            .into_iter()
+                            .map(|(name, val)| (String::from_utf8_lossy(&name).to_string(), val))
+                            .collect(),
+                        span,
+                    ),
+                )]
+            };
 
             return (
-                ResolvedImportPattern::new(
-                    decls,
-                    vec![(final_name.clone(), self_id)],
-                    vec![(final_name, const_record)],
-                ),
+                ResolvedImportPattern::new(decls, vec![(final_name.clone(), self_id)], constants),
                 errors,
             );
         };

tests/const_/mod.rs

@@ -238,13 +238,30 @@ fn complex_const_export() {
     let actual = nu!(&inp.join("; "));
     assert_eq!(actual.out, "eats");
 
-    let inp = &[
-        MODULE_SETUP,
-        "use spam",
-        "($spam.eggs.bacon.none | is-empty)",
-    ];
+    let inp = &[MODULE_SETUP, "use spam", "'none' in $spam.eggs.bacon"];
     let actual = nu!(&inp.join("; "));
-    assert_eq!(actual.out, "true");
+    assert_eq!(actual.out, "false");
+}
+
+#[test]
+fn only_nested_module_have_const() {
+    let setup = r#"
+        module spam {
+            export module eggs {
+                export module bacon {
+                    export const viking = 'eats'
+                    export module none {}
+                }
+            }
+        }
+    "#;
+    let inp = &[setup, "use spam", "$spam.eggs.bacon.viking"];
+    let actual = nu!(&inp.join("; "));
+    assert_eq!(actual.out, "eats");
+
+    let inp = &[setup, "use spam", "'none' in $spam.eggs.bacon"];
+    let actual = nu!(&inp.join("; "));
+    assert_eq!(actual.out, "false");
 }
 
 #[test]
@@ -261,20 +278,16 @@ fn complex_const_glob_export() {
     let actual = nu!(&inp.join("; "));
     assert_eq!(actual.out, "eats");
 
-    let inp = &[MODULE_SETUP, "use spam *", "($eggs.bacon.none | is-empty)"];
+    let inp = &[MODULE_SETUP, "use spam *", "'none' in $eggs.bacon"];
     let actual = nu!(&inp.join("; "));
-    assert_eq!(actual.out, "true");
+    assert_eq!(actual.out, "false");
 }
 
 #[test]
 fn complex_const_drill_export() {
-    let inp = &[
-        MODULE_SETUP,
-        "use spam eggs bacon none",
-        "($none | is-empty)",
-    ];
+    let inp = &[MODULE_SETUP, "use spam eggs bacon none", "$none"];
     let actual = nu!(&inp.join("; "));
-    assert_eq!(actual.out, "true");
+    assert!(actual.err.contains("variable not found"));
 }
 
 #[test]

tests/repl/test_modules.rs

@@ -120,6 +120,11 @@ fn export_consts() -> TestResult {
     )
 }
 
+#[test]
+fn dont_export_module_name_as_a_variable() -> TestResult {
+    fail_test(r#"module spam { }; use spam; $spam"#, "variable not found")
+}
+
 #[test]
 fn func_use_consts() -> TestResult {
     run_test(