def-node carried :meta nil when a def had no metadata, fn-node carried :name
nil for anonymous fns, analyze-arity carried :rest nil for fixed arities, and
empty-env carried :recur nil. These all mean genuine absence, and every
consumer reads an absent key back as nil — so writing the nil is gratuitous.
Dropping them keeps the common nodes (defs, fns, fixed arities) nil-free, so
once user maps preserve nil they stay plain structs instead of becoming phm.
Faithful nil values (a nil const :val) are left as-is; the back end handles
those. No-op today since structs already drop nil: suite, conformance 218/218
x3, fixpoint, fib all unchanged.
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