Fixed a bug in Python source term with PETSc

wenqing requested to merge wenqing/ogs:fixing into master

For parallel computing with DDC, a partition may not have source term but a nullptr is assigned for it in the source term vector. That nullptr element of the source term vector causes ogs crashes when the follow function is called:

void SourceTermCollection::integrate(const double t, GlobalVector const& x,
                                     GlobalVector& b, GlobalMatrix* jac) const
    for (auto const& st : _source_terms)
        st->integrate(t, x, b, jac);

This MR fixes that bug.

  1. Feature description was added to the changelog
  2. Tests covering your feature were added?
  3. Any new feature or behavior change was documented?

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