help generate_docs | flatten crashes nushell (#3099)

* fix case where parent_name was {nu, term} and possibly others in the future by doing an extra test first to see if if the *parent_name key actually exists in cmap

* update with help generate_docs testing

crates/nu-command/tests/commands/help.rs

@@ -0,0 +1,31 @@
+use nu_test_support::{nu, pipeline};
+
+#[test]
+fn help_commands_count() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        help commands | count
+        "#
+    ));
+
+    let output = actual.out;
+    let output_int: i32 = output.parse().unwrap();
+    let is_positive = output_int.is_positive();
+    assert!(is_positive);
+}
+
+#[test]
+fn help_generate_docs_count() {
+    let actual = nu!(
+        cwd: ".", pipeline(
+        r#"
+        help generate_docs | flatten | count
+        "#
+    ));
+
+    let output = actual.out;
+    let output_int: i32 = output.parse().unwrap();
+    let is_positive = output_int.is_positive();
+    assert!(is_positive);
+}