fix: replace truthiness checks with is not None in PlasmodiumDataResource._subset_genome_sequence_region

[Gopisokk]

What problem does this solve?

_subset_genome_sequence_region() in plasmodium.py uses truthiness checks (if region.start: / if region.end:) to decide whether to slice the genome sequence. Because Python evaluates 0 as falsy, specifying region.start = 0 silently skips slicing and returns the entire contig instead of the requested sub-region.

This is the same class of bug as #940, but in the Plasmodium codebase (PlasmodiumDataResource) which is not covered by that issue.

How does it solve it?

Replace the truthiness checks with explicit None checks:

-        if region.start:
+        if region.start is not None:
             slice_start = region.start - 1
         else:
             slice_start = None
-        if region.end:
+        if region.end is not None:
             slice_stop = region.end
         else:
             slice_stop = None

This correctly distinguishes between "no boundary specified" (None) and "boundary is zero" (0).

Also fixes a docstring typo: Sebset → Subset.

Relevant issues

Related: #940

Testing done

• Pre-commit hooks (ruff lint, ruff format, trailing whitespace, end of files) all pass ✅

Breaking changes

None. This fix restores the expected behavior for edge-case inputs that were previously silently mishandled.
Closes #1108