fix(erlang): distinguish empty optional BEAM chunks

Select legacy Atom when AtU8 is empty and skip zero-byte optional FunT. Preserve explicitly empty counted arrays, malformed nonempty failures, last-occurrence selection and original chunk provenance. Share structural fixtures across producer and public workflow regressions.

Fixes #6816
Fixes #6874

docs/erlang-beam.md

@@ -31,7 +31,7 @@ The BEAM form size bounds the parsed chunks. Bytes after the declared form are
 ignored by the parser but remain part of the selected artifact's byte length
 and SHA-256.
 
-When `AtU8` or `Atom` is present, `atoms` is the 1-based name table and
+When a selected `AtU8` or `Atom` table is decoded, `atoms` is the 1-based name table and
 `module` is atom 1. Both the pre-OTP 28 table and the OTP 28+ compact table
 (negated count, compact-encoded name lengths) are read. `exports`, `imports`, and `lambdas` resolve atom indices to
 those names when the table is present; unresolved indices keep `function` or
@@ -41,6 +41,19 @@ those names when the table is present; unresolved indices keep `function` or
 function count, and the instruction stream's file range. Missing optional
 tables are null, not empty-everywhere.
 
+A zero-byte `AtU8` falls back to an available legacy `Atom` table, following
+[OTP 20.3's atom selection](https://github.com/erlang/otp/blob/a113f6117fd696ea6f84ed754055b4ec97a7ccb2/erts/emulator/beam/beam_load.c).
+A nonempty `AtU8` retains UTF-8 precedence, including an explicit zero atom
+count; malformed nonempty data fails instead of falling back. An empty atom
+chunk without an alternate table still fails its required count header.
+Absent and zero-byte optional `FunT` chunks produce `lambdas: null`, while a
+four-byte zero row count produces `lambdas: []`. This follows
+[OTP 29's positive-size lambda parsing rule](https://github.com/erlang/otp/blob/550d7b7898706c7822362c42e7b93120c1d1f29a/erts/emulator/beam/beam_file.c).
+Empty payloads retain their ranges and SHA-256 in the chunk inventory. These
+rules apply to the last occurrence of each chunk name; an empty final occurrence
+does not revive an earlier payload of that name. Nonempty truncated tables remain
+format failures.
+
 ## Limits and failures
 
 The selected file is limited to 64 MiB by default (512 MiB maximum). At most