Return empty list on invalid globs

Regression was introduced in 9479c28.

Since that commit providing an invalid glob to tera will make it fail to
create an instance, first (in that commit) by making it panic and since
1f95878 by making it return an error.

While returning an error is not entirely unlogical, it doesn't match
what most languages do with invalid globs.

- Bash will by default return an empty set on invalid globs, as the
  `failglob` option is off by default
- Python will likewise return the empty set instead of throwing an
  exception, when doing something like `glob.glob("/dev/null/*")`
- Rust's `globwalk` will also not error, but return an empty set

In fact, we use globwalk in tera and the only reason we panic is by
accident, because `std::fs::canonicalize()` checks the path.

It is better to match other language's glob behaviour, therefore we
resort back to the original path if `canonicalize()` fails to return the
empty set again when encountering an invalid glob.

Even more so, since this caused a lot of regressions already, including
in zola.

Fixes getzola/zola#2150

ref: Keats#819
ref: Keats#820
ref: Keats#799

src/tera.rs

@@ -132,7 +132,13 @@ impl Tera {
         // See https://github.com/Keats/tera/issues/574 for the Tera discussion
         // and https://github.com/Gilnaa/globwalk/issues/28 for the upstream issue.
         let (parent_dir, glob_end) = glob.split_at(glob.find('*').unwrap());
-        let parent_dir = std::fs::canonicalize(parent_dir)?;
+        let parent_dir = match std::fs::canonicalize(parent_dir) {
+            Ok(d) => d,
+            // If canonicalize fails, just abort it and resume with the given path.
+            // Consumers expect invalid globs to just return the empty set instead of failing.
+            // See https://github.com/Keats/tera/issues/819#issuecomment-1480392230
+            Err(_) => std::path::PathBuf::from(parent_dir),
+        };
         let dir = parent_dir.join(glob_end).into_os_string().into_string().unwrap();
 
         // We are parsing all the templates on instantiation
@@ -1222,9 +1228,10 @@ mod tests {
 
     // https://github.com/Keats/tera/issues/819
     #[test]
-    fn doesnt_panic_on_invalid_glob() {
+    fn empty_list_on_invalid_glob() {
         let tera = Tera::new("\\dev/null/*");
         println!("{:?}", tera);
-        assert!(tera.is_err());
+        assert!(tera.is_ok());
+        assert!(tera.unwrap().templates.is_empty());
     }
 }