fix: raise ValueError on empty CNV HMM data instead of IndexError

[ADAMSmugwe]

Problem

Two plotting functions in cnv_data.py crash with a cryptic
IndexError: index 0 is out of bounds for axis 0 with size 0
when a requested region or sample has no valid CNV HMM data:

• plot_cnv_hmm_coverage — after data.query("call_CN >= 0") filters
  out all uncalled windows, accessing values[0] / values[-1] on the
  empty DataFrame crashes.
• plot_cnv_hmm_heatmap_track — if n_windows == 0 for the requested
  region, accessing start[0] and end[-1] crashes.

Fix

Added an early ValueError guard in both functions before the unsafe
index access, with a descriptive message that names the sample/region
so the user knows exactly what produced no data.

Follows the existing pattern used in frq_base.py and elsewhere in
the codebase.

Files Changed

• malariagen_data/anoph/cnv_data.py