Fix contour levels for TC density map

[chengzhuzhang]

Description

In the recent two zppy weekly test, it appears that upgrading matplotlib from 3.9 to 3.10 has shifted behavior of the TC density map, as shown in below, 0 values are not represented correctly with current contour levels, as shown below. Slightly update clevels can fix this issue.

When fixing this issue, I discovered another problem that can miss one storm count for the period being analyses/ or cause crash if only single storm present in the tracking data. This is also fixed by this PR.

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules

If applicable:

• New and existing unit tests pass with my changes (locally and CI/CD build)
• I have added tests that prove my fix is effective or that my feature works
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have noted that this is a breaking change for a major release (fix or feature that would cause existing functionality to not work as expected)

[chengzhuzhang]

@tomvothecoder this is a minor change, let me know if you spot any potential problem. Thanks!

[tomvothecoder]

LGTM