fix(tvix/eval): null in dynamic attribute keys skips the element

This is actually *tested* behaviour in C++ Nix, so we need to
implement it here, too.

Change-Id: Ic4a4659a2f04cdd928cbe78a85dae90401515371
Reviewed-on: https://cl.tvl.fyi/c/depot/+/6199
Reviewed-by: sterni <sternenseemann@systemli.org>
Tested-by: BuildkiteCI

tvix/eval/src/value/attrs.rs

@@ -203,6 +203,13 @@ impl NixAttrs {
                     )?;
                 }
 
+                Value::Null => {
+                    // This is in fact valid, but leads to the value
+                    // being ignored and nothing being set, i.e. `{
+                    // ${null} = 1; } => { }`.
+                    continue;
+                }
+
                 other => {
                     return Err(Error::InvalidKeyType {
                         given: other.type_of(),