Fix(pw): suppress spurious warning when no plane waves exist for a k-point

[Cstandardlib]

Linked Issue

Refs #7201

Description

This PR fixes the spurious no plane waves warning that occur during SCF calculations when certain k-points have zero plane waves due to insufficient wavefunction cutoff energy (ecutwfc).

When performing calculations with small simulation cells and low energy cutoffs,
k-points located far from reciprocal lattice points (requiring large |k+G| vectors)
may fall outside the kinetic energy cutoff sphere,
resulting in zero plane waves for those k-points. In such regimes:

• At low ecutwfc (50–60 Ry), the code immediately aborted with "no plane waves".
• At medium ecutwfc (70–130 Ry), plane waves existed but npwx < nbands,
  leading to rank deficiency and psi_norm <= 0 during CG diagonalization in diago_cg.cpp.

These k-points with zero plane waves are valid edge cases. This PR adds the
necessary zero-count guards so the code reports energy/k-points issue instead of reducing the cores.

What's Changed

• Fix the warning message logic in PW_Basis_K::setuptransform() when no plane waves are available for a k-point.
• Distinguish between two cases and emit appropriate warnings:
  1. Globally empty: No plane waves exist for this k-point across the entire MPI pool.
     Prompts the user to increase ecutwfc or check KPT settings.
  2. Locally empty (parallel): The current MPI rank has zero plane waves, but other ranks in the pool still have some.
     Prompts the user to reduce the number of cores.

Unit Tests and/or Case Tests for my changes

• Add two unit tests to cover both scenarios:
  • test_no_plane_wave_message_global_empty_k
  • test_no_plane_wave_message_parallel_local_empty

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