Fix #115715: Boolean behaves as if the object is on it's origin

Resolve regression in [0] which replaced a null check with an
identity comparison. This wasn't correct as the null check indicated
that transform was applied or not, where situations that require a
transform could have an identity matrix.

Resolve by checking if the matrix that would transform vertices is an
identity which is a reliable way to know if any transformation is needed.

Ref: !115736

[0]: e24c7f1954

source/blender/blenkernel/intern/mesh_boolean_convert.cc

@@ -316,8 +316,8 @@ static IMesh meshes_to_imesh(Span<const Mesh *> meshes,
     /* Allocate verts
      * Skip the matrix multiplication for each point when there is no transform for a mesh,
      * for example when the first mesh is already in the target space. (Note the logic
-     * directly above, which uses an identity matrix with a null input transform). */
-    if (obmats.is_empty() || obmats[mi] == float4x4::identity()) {
+     * directly above, which uses an identity matrix with an empty input transform). */
+    if (obmats.is_empty() || r_info->to_target_transform[mi] == float4x4::identity()) {
       threading::parallel_for(vert_positions.index_range(), 2048, [&](IndexRange range) {
         for (int i : range) {
           float3 co = vert_positions[i];