fix: minor time widget fix

[fdrgsp]

Fixing the _create_new_row method in TimePlanWidget so that we can pass 0.0 as a possible interval: adding if isinstance(interval, timedelta) instead of only if interval.

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (90706e6) 89.39% compared to head (d6fc795) 89.39%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #153   +/-   ##
=======================================
  Coverage   89.39%   89.39%           
=======================================
  Files          37       37           
  Lines        4999     4999           
=======================================
  Hits         4469     4469           
  Misses        530      530           

Files Changed	Coverage Δ
src/pymmcore_widgets/_mda/_time_plan_widget.py	96.35% <100.00%> (ø)

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