Xml errors fix (#11487)

# Description
Fixes #11264
This PR adds checks in `to xml` to output error for malformed xml
entries:
* With columns that are not one of `tag`, `attributes` or `content`
* With no `tag` when entry is not a string
* With `tag` that is not a string
This PR also replaces `attrs` with `attributes` in example and
extra_usage of `to xml` (column was originally named attrs and renamed
to attributes, but this was missed in docs)

# User-Facing Changes
`to xml` will produce error for conditions described above instead of
silently returning nothing

# Tests + Formatting
Added tests for `to xml` to check handling of malformed xml entries

crates/nu-command/tests/format_conversions/xml.rs

@@ -22,3 +22,39 @@ fn table_to_xml_text_and_from_xml_text_back_into_table() {
 
     assert_eq!(actual.out, "true");
 }
+
+#[test]
+fn to_xml_error_unknown_column() {
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+            {tag: a bad_column: b} | to xml
+        "#
+    ));
+
+    assert!(actual.err.contains("Invalid column \"bad_column\""));
+}
+
+#[test]
+fn to_xml_error_no_tag() {
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+            {attributes: {a: b c: d}} | to xml
+        "#
+    ));
+
+    assert!(actual.err.contains("Tag missing"));
+}
+
+#[test]
+fn to_xml_error_tag_not_string() {
+    let actual = nu!(
+        cwd: "tests/fixtures/formats", pipeline(
+        r#"
+            {tag: 1 attributes: {a: b c: d}} | to xml
+        "#
+    ));
+
+    assert!(actual.err.contains("not a string"));
+}