RFR: 8341336: Fix -Wzero-as-null-pointer-constant warnings in PRODUCT-only codefix warnings in release build

[Kim Barrett]

Please review this change that removes -Wzero-as-null-pointer-constant warnings
triggered by PRODUCT-only code. Most are uses of PRODUCT_RETURN0 in a pointer
context.  These are changed to use the (new) macro PRODUCT_RETURN_NULL.

Testing: mach5 tier1
Locally (linux-x64) ran tier1 testing on a release build.
(mach5 mostly does fastdebug testing)

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Commit messages:
 - fix warnings in release build

Changes: https://git.openjdk.org/jdk/pull/21290/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21290&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341336
  Stats: 14 lines in 5 files changed: 4 ins; 4 del; 6 mod
  Patch: https://git.openjdk.org/jdk/pull/21290.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21290/head:pull/21290

PR: https://git.openjdk.org/jdk/pull/21290