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.

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(