RFR: Fix nullability signature attr tests

[Vicente Romero]

fixing regression tests related to signatures with null restricted elements

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Commit messages:
 - other tests
 - Merge branch 'bworld' into fix.NullabilitySignatureAttrTests
 - additional changes
 - Merge branch 'bworld' into fix.NullabilitySignatureAttrTests
 - fix test NullabilitySignatureAttrTests.java

Changes: https://git.openjdk.org/valhalla/pull/1887/files
  Webrev: https://webrevs.openjdk.org/?repo=valhalla&pr=1887&range=00
  Stats: 279 lines in 1 file changed: 3 ins; 237 del; 39 mod
  Patch: https://git.openjdk.org/valhalla/pull/1887.diff
  Fetch: git fetch https://git.openjdk.org/valhalla.git pull/1887/head:pull/1887

PR: https://git.openjdk.org/valhalla/pull/1887