[rust-compiler] Skip empty optional ScopeInfo maps on serialize

[poteto]

bash compiler/scripts/test-babel-ast.sh runs three sub-tests in
order: round_trip_all_fixtures, scope_info_round_trip, and
scope_resolution_rename. The first and third are green after the
earlier slices in this stack, but the middle one fails 1780/1780
with diffs of the shape:

--- compiler/<fixture>.json ---
Round-trip mismatch:
+   "nodeIdToScope": {},

This was previously masked because the round-trip failure earlier in
the script tripped set -e and the scope-info test never ran. With
round-trip green, it now surfaces a pre-existing serialization
mismatch in ScopeInfo.

Root cause: the Babel-side fixture producer in
compiler/scripts/babel-ast-to-json.mjs::collectScopeInfo emits only
five keys per .scope.json file — scopes, bindings,
nodeToScope, referenceToBinding, and programScope. It never
writes nodeIdToScope, refNodeIdToBinding, or nodeToScopeEnd.

The Rust ScopeInfo struct declares those three fields with
#[serde(default)], so deserialization happily produces empty
HashMap / IndexMap values when the keys are missing. Serialization
was then emitting them as "nodeIdToScope": {} (etc.), which the
round-trip diff treats as a mismatch.

Add skip_serializing_if = "<Map>::is_empty" to all three fields so
a fixture that came in without the key goes back out without the
key. Required fields (no #[serde(default)]: scopes, bindings,
node_to_scope, reference_to_binding, program_scope) are
deliberately untouched — they are present in every fixture by
design.

Alternatives considered:

• Remove #[serde(default)] from the three fields. Rejected: would
  break deserialization of legacy fixtures that legitimately omit
  these supplemental maps.
• Wrap the maps in Option<...>. Rejected: changes field types and
  forces every reader to unwrap, with no behavior win over the
  existing default + skip-on-empty combination.
• Custom skip function. Rejected: HashMap::is_empty and
  IndexMap::is_empty express intent directly and are already in
  scope via the existing imports.
• Add a Default impl for ScopeInfo. Rejected: the issue is on
  the serialize side, not deserialize; serde(default) already
  supplies empty maps on deserialize correctly.

Test plan:

• bash compiler/scripts/test-babel-ast.sh:
  Before: round_trip 1779/1779, scope_info 0/1780 (FAIL), rename 1779/1779; exit 101
  After: round_trip 1779/1779, scope_info 1780/1780, rename 1779/1779; exit 0
• cargo build: clean across the workspace.