Fix default value of keyword parameter saveat

[ytdHuang]

Checklist

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Description

The original default value of saveat is given by

saveat = e_ops isa Nothing ? t_l : [t_l[end]]

This causes ambiguity when user specify e_ops = [], because:

[] isa Nothing # false

Therefore, I change it to

saveat = is_empty_e_ops ? t_l : [t_l[end]]

where is_empty_e_ops is pre-defined as

is_empty_e_ops = isempty(e_ops)

and this works becasue

isempty([]) # ture
isempty(nothing) # true

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.54%. Comparing base (87ce64e) to head (586b910).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #260      +/-   ##
==========================================
- Coverage   93.16%   92.54%   -0.63%     
==========================================
  Files          31       31              
  Lines        2327     2321       -6     
==========================================
- Hits         2168     2148      -20     
- Misses        159      173      +14     

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[albertomercurio]

LGTM!

[albertomercurio]

Oh, wait, the code quality failed

[albertomercurio]

But it failed for other reasons. Strange.

[albertomercurio]

Ok, it is a problem related to the new release of Julia v1.11.0. Let's merge it