feat(tvix/eval): add builtins.isNull

Change-Id: Iae251d41b4ac6b77df56078a954ec3e33b7f9ccf
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6256
Tested-by: BuildkiteCI
Reviewed-by: sterni <sternenseemann@systemli.org>

tvix/eval/src/builtins/mod.rs

@@ -17,6 +17,11 @@ macro_rules! builtin {
 pub fn global_builtins() -> HashMap<&'static str, Value> {
     let mut globals = HashMap::new();
 
+    builtin!(globals, "isNull", 1, |args| Ok(Value::Bool(matches!(
+        args[0],
+        Value::Null
+    ))));
+
     builtin!(globals, "toString", 1, |args| {
         // TODO: toString is actually not the same as Display
         Ok(Value::String(format!("{}", args[0]).into()))