fix(gpu): guard qq synchronization without projectors

[GenZeal-Lin]

Linked Issue

Fixes #7681

What's changed?

For pseudopotentials without nonlocal projectors, nhm is zero and
the corresponding qq buffers are not allocated.

The previous GPU initialization path still attempted to synchronize
these qq buffers, which could pass an invalid destination pointer to
CUDA memory-copy operations and trigger cudaMemcpy invalid argument.

This change guards the synchronization of s_qq_nt, c_qq_so,
z_qq_so, and d_qq_nt with nhm > 0.

The existing path for pseudopotentials with nonlocal projectors
(nhm > 0) is unchanged.

Verification

• CUDA build on NVIDIA RTX 4090: passed.
• 001_PW_UPF100_Al (nhm > 0) and
  016_PW_BLPS–019_PW_Coulomb (nhm == 0) passed the core
  default-precision regression tests.
• Single-precision smoke tests for both nhm == 0 and nhm > 0
  completed successfully.
• Extended cases 801–814 no longer show the CUDA invalid-argument crash.
• Case 803 retains the previously investigated small stress deviation,
  which is related to MPI/reference configuration rather than this fix.

Scope

• Modified:
  source/source_pw/module_pwdft/vnl_pw.cpp
• No INPUT parameter changes.
• No API changes.

[mohanchen]

LGTM. Thanks for your contribution!