[C++] Fix NULL pointer dereference in reflection VerifyVector() (union vector, CWE-476)

[thinhnallf]

Bug

In src/reflection.cpp, the reflection::Union case inside VerifyVector() can encounter a null pointer dereference if type_vec is null.

auto type_vec = table.GetPointer<Vector<uint8_t>*>(vec_field.offset() - sizeof(voffset_t));
if (!v.VerifyVector(type_vec)) return false; 
if (type_vec->size() != vec->size()) return false;  // NULL DEREF if type_vec is null

Since VerifierTemplate::VerifyVector(nullptr) intentionally returns true (as a null vector represents a valid, absent optional field), a null type_vec safely passes the verification line but subsequently causes an unconditional null pointer dereference during the size comparison.

Fix

Add an explicit null-check for type_vec immediately after the verification check, before it is dereferenced.

Testing

Confirmed locally that handling an absent companion type-vector field no longer causes a crash and is now correctly rejected by returning false.

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[thinhnallf]

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[thinhnallf]

@dbaileychess friendly ping on this one. CI is currently "waiting for status"
because the required workflows need a maintainer's approval for a first-time
contributor. This is a small, self-contained fix (one guard line) plus a
regression test with a negative control for a NULL-deref in the reflection
verifier's vector-of-unions path (related to, but distinct from, #8567).
Happy to adjust anything. Thanks!