templates: Be strict about undefined variables

We shouldn't have any reason to use undefined variables in MAS, so
silently printing as empty strings (etc) seems undesirable.

This will also be helpful for operators with custom templates, since
then they will notice their templates being broken.

Author: reivilibre

crates/templates/src/lib.rs

@@ -17,7 +17,7 @@ use camino::{Utf8Path, Utf8PathBuf};
 use mas_i18n::Translator;
 use mas_router::UrlBuilder;
 use mas_spa::ViteManifest;
-use minijinja::Value;
+use minijinja::{UndefinedBehavior, Value};
 use rand::Rng;
 use serde::Serialize;
 use thiserror::Error;
@@ -205,6 +205,8 @@ impl Templates {
             span.in_scope(move || {
                 let mut loaded: HashSet<_> = HashSet::new();
                 let mut env = minijinja::Environment::new();
+                // Don't allow use of undefined variables
+                env.set_undefined_behavior(UndefinedBehavior::Strict);
                 let root = path.canonicalize_utf8()?;
                 info!(%root, "Loading templates from filesystem");
                 for entry in walkdir::WalkDir::new(&root)