Avoid nonnull warning in rvalue construction of TfSmallVector without local storage

[nvmkuruc]

Description of Change(s)

TfSmallVector supports instantiations with no local storage (the capacity template argument set to 0). These are utilized in pxr/exec.

The TfSmallVector move constructor assumes is that if the sizes of the local storage and the rvalue parameter are the same, the rvalue's data can be moved into the constructing object's local storage. However, if the local storage size is 0, the _UninitializedMove tries to move an empty range into non-existent local storage. In newer versions of GCC, this causes a warning.

# Truncated stack trace
inlined from ‘static Iterator pxrInternal_v0_26_5__pxrReserved__::TfSmallVectorBase::_UninitializedMove(Iterator, Iterator, Iterator) [with Iterator = pxrInternal_v0_26_5__pxrReserved__::GfQuatd*]’ at pxr/base/tf/smallVector.h:84:39,
inlined from ‘pxrInternal_v0_26_5__pxrReserved__::TfSmallVector<T, N>::TfSmallVector(pxrInternal_v0_26_5__pxrReserved__::TfSmallVector<T, N>&&) [with T = pxrInternal_v0_26_5__pxrReserved__::GfQuatd; unsigned int N = 0]’ at pxr/base/tf/smallVector.h:245:35
...
error: argument 1 null where non-null expected [-Werror=nonnull]

This change introduces a constexpr guard to only apply the move if local storage is available.

With this change, the following build command should build without warnings or errors in GCC 13.

python3 build_scripts/build_usd.py ../inst/usd-strict --no-imaging --no-examples --no-materialx  --build-args USD,'-DPXR_STRICT_BUILD_MODE=ON -DCMAKE_CXX_FLAGS="-Wno-array-bounds -Wno-uninitialized"' -vv --tests

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Fixes Issue(s)

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:
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• I have verified that all unit tests pass with the proposed changes

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[sunyab]

Filed as internal issue #USD-12078

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