RFR: 8337302: Undefined type variable results in null

[Rafael Winterhalter]

When a type uses a type variable without a declaration, no exception is thrown. This change triggers a `TypeNotFoundException` to be thrown.

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Commit messages:
 - JDK-8337302: Throw exception upon search for type variable without representation.

Changes: https://git.openjdk.org/jdk/pull/20535/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20535&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8337302
  Stats: 84 lines in 3 files changed: 74 ins; 0 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/20535.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20535/head:pull/20535

PR: https://git.openjdk.org/jdk/pull/20535