fix: add null guard for writeEnum and enumCapacity in JSONB.IO (#7806)

[waterWang]

Summary

Fixes #7806: NPE when serializing an object with a null enum field via JSONB.toBytes with WriteNulls (and WriteClassName).

Root cause

When JSONB.toBytes(obj, WriteNulls, ...) runs on a class with an enum field whose value is null, the ASM-generated ObjectWriter (class OWG_*) serializes the field through JSONB.IO.writeEnum(byte[], int, Enum, long) directly. That method had no null guard — with e == null it accessed e.ordinal() and threw NullPointerException.

java.lang.NullPointerException
  at com.alibaba.fastjson2.JSONB$IO.writeEnum(JSONB.java:2196)
  at com.alibaba.fastjson2.writer.OWG_1_5_ResultValue.writeJSONB
  at com.alibaba.fastjson2.JSONB.toBytes(JSONB.java:1836)

The companion JSONB.IO.enumCapacity (used for buffer pre-allocation) had the same missing guard.

FieldWriterEnum.writeEnumValueJSONB already handled null correctly (writes BC_NULL), but the ASM fast path bypassed it when symbolTable is null.

Fix

Add early null checks to both methods in JSONB.IO:

• writeEnum: write BC_NULL (1 byte) when e is null, return off + 1
• enumCapacity: return 1 (the BC_NULL size) when e is null

This matches the existing null handling in FieldWriterEnum.writeEnumValueJSONB and JSONWriter.writeEnum.

Verification

Reproduced the exact stack trace on main, then confirmed the fix:

• null enum + WriteNulls + WriteClassName → serializes color:null (previously NPE)
• non-null enum unchanged
• JSON text path unaffected

One file changed (+7). No functional change for non-null enum values.

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[wenshao]

Reviewed. Suggestions are inline.

— gpt-5.6-sol@954e5164 via Qwen Code /review (v0.22.0)

[wenshao]

[Suggestion] Add a regression test for the ASM null-enum JSONB path

This fixes the production path, but no test pins the generated OWG_* direct writer that triggered issue 7806. A later refactor could remove or bypass either guard and JSONB.toBytes(bean, WriteNulls, WriteClassName) would again throw while the current suite remains green.

An isolated probe confirmed the generated writer was OWG_1_1_Bean; removing the writeEnum guard produced:

java.lang.NullPointerException: Cannot invoke "java.lang.Enum.ordinal()" because "e" is null
 at com.alibaba.fastjson2.JSONB$IO.writeEnum(JSONB.java:2199)
 at com.alibaba.fastjson2.writer.OWG_1_1_Bean.writeJSONB(Unknown Source)

Please add an Issue7806 regression test with a null enum field through the ASM writer, asserting WriteNulls + WriteClassName preserves null and a non-null value remains unchanged. Add a WriteEnumsUsingName or WriteEnumUsingToString variant to independently pin enumCapacity's guard. The test must fail when the writeEnum guard is removed, and the enum-name/toString variant must fail when the enumCapacity guard is removed.

— gpt-5.6-sol@954e5164 via Qwen Code /review (v0.22.0)