RFR: 8310905: [lw5] addressing review comments on null restricted types

[Vicente Romero]

addressing some review comments related to PR [1]

[1] https://github.com/openjdk/valhalla/pull/872

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Commit messages:
 - 8310905: [lw5] addressing review comments on null restricted types

Changes: https://git.openjdk.org/valhalla/pull/880/files
 Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=880&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8310905
  Stats: 63 lines in 6 files changed: 30 ins; 12 del; 21 mod
  Patch: https://git.openjdk.org/valhalla/pull/880.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/880/head:pull/880

PR: https://git.openjdk.org/valhalla/pull/880