exec: silence compiler warnings by providing a default value

[davvid]

Although exec uses a zero allocator, gcc (Debian 15.3.0-1) 15.3.0 emits warnings for the Exec_NodeRecompilationInfoTable::_Storage struct because it does not have visibility into these details.

GCC warns that the struct is used uninitialized in the expansion of !TF_VERIFY(!storage->isInfoConstructed, ...).

Provide a default value to silence the diagnostic.

Description of Change(s)

Fixes Issue(s)

• Compilation warnings with GCC 15 and others.

Checklist

• I have created this PR based on the dev branch
• I have followed the coding conventions
• I have added unit tests that exercise this functionality (Reference:
  testing guidelines)
• I have verified that all unit tests pass with the proposed changes
• I have submitted a signed Contributor License Agreement (Reference:
  Contributor License Agreement instructions)

[jesschimein]

Filed as internal issue #USD-12501

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