fix: guard against empty data in plot_cnv_hmm_coverage_track

[suhr25]

SUMMARY

This PR fixes a crash in plot_cnv_hmm_coverage_track when filtering leaves no data. Instead of throwing an error, the function now safely returns None.

The change is small and limited to malariagen_data/anoph/cnv_data.py.

---

FIX

Before:

data = data.query("call_CN >= 0")

x_min = data["variant_position"].values[0]  # crashes if empty

After:

data = data.query("call_CN >= 0")

if len(data) == 0:
    return None

---

VERIFICATION

Run the function on a region where all HMM calls are filtered out. Previously, this caused a crash. Now, it returns None cleanly.
Normal cases still behave exactly the same.

[suhr25]

Hi @jonbrenas,
This PR fixes a crash in plot_cnv_hmm_coverage_track, kindly take a look.
Thanks!