analyze-try assoc'd :catch-sym/:catch-body/:finally nil-when-absent, so a try
with no catch (or no finally) carried a nil-valued key — which makes the node a
phm in jolt's map representation and forces the back end to densify it
(norm-node) before reading :op. That's the map-nil-representation trap Phase 2
already cleaned up for def/fn/arity nodes. Add those keys only when the clause is
present, matching the arity :rest discipline; a try node stays a fast struct.
Behavior-invisible: emit-try reads each key with a nil-safe (node :k) and gates
on it, so an absent key and a present-nil key are indistinguishable to every
consumer. Adds ir-try-shape-test asserting the node shape across all four
try/catch/finally combinations plus end-to-end eval.
Note on scope: the plan's "delete the defensive norm-node calls" is NOT done — it
can't be. {:op :const :val nil} (e.g. (def x nil)) and nil map keys are
inherently phm, so the emit-dispatch norm-node guards a real case, not a
present-or-absent artifact. This PR removes a source of gratuitous phm nodes
rather than the densification itself. Full gate green.
