RFR: 8335593: Fix -Wzero-as-null-pointer-constant warning in Type_Array ctor

[Kim Barrett]

Please review this trivial change to the Type_Array constructor. The initial
value for the _types member is changed from 0 to nullptr. This removes some
-Wzero-as-null-pointer-constant warnings when building with that enabled.

Testing: mach5 tier1

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Commit messages:
 - fix Type_Array ctor

Changes: https://git.openjdk.org/jdk/pull/19998/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19998&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8335593
  Stats: 2 lines in 1 file changed: 0 ins; 0 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/19998.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19998/head:pull/19998

PR: https://git.openjdk.org/jdk/pull/19998