RFR: 8335369: Fix -Wzero-as-null-pointer-constant warnings in ImmutableOopMapBuilder

[Kim Barrett]

On Mon, 1 Jul 2024 04:17:30 GMT, Julian Waters <jwaters at openjdk.org> wrote:

> Doesn't seem like it's too much harm in setting the map parameter's default to nullptr, but oh well

All callers have a map that they pass as that argument, and it's never null.  And semantically the operation is about
setting the map at a certain location, so a default doesn't seem to make sense.

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PR Comment: https://git.openjdk.org/jdk/pull/19961#issuecomment-2201998011