RFR: 8342700: Set the default value of DirectCodeBuilder::handlers to Null

[Shaojin Wen]

Set the default value of DirectCodeBuilder::handlers to Null to reduce overhead in scenarios where there is no handler.

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Commit messages:
 - handlers default null

Changes: https://git.openjdk.org/jdk/pull/21598/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21598&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342700
  Stats: 19 lines in 3 files changed: 14 ins; 0 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/21598.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21598/head:pull/21598

PR: https://git.openjdk.org/jdk/pull/21598