RFR: 8338874: [lw5] add nullable types

[Vicente Romero]

adding support for nullable types

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Commit messages:
 - 8338874: [lw5] add nullable types

Changes: https://git.openjdk.org/valhalla/pull/1219/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1219&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338874
  Stats: 329 lines in 6 files changed: 214 ins; 72 del; 43 mod
  Patch: https://git.openjdk.org/valhalla/pull/1219.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1219/head:pull/1219

PR: https://git.openjdk.org/valhalla/pull/1219