Fix geopandas type check

[ahuang11]

If geometry is None, it crashes:

import hvplot.pandas
import censusdis.data as ced
from censusdis.datasets import ACS5
from censusdis.counties.washington import KING
from censusdis.states import WA

GROUP = "B25008"

gdf_king = ced.download(
    ACS5,
    2020,
    # Instead of listing the variables, we can ask for a whole group.
    group=GROUP,
    state=WA,
    county=KING,
    tract="*",
    with_geometry=True,
    # An optional flag that produces better maps in coastal areas.
    # More on this in the next lesson.
    remove_water=True,
)
gdf_king

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 88.41%. Comparing base (6c96c7e) to head (79a13dd).
Report is 6 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1367      +/-   ##
==========================================
+ Coverage   87.39%   88.41%   +1.02%     
==========================================
  Files          50       50              
  Lines        7490     7366     -124     
==========================================
- Hits         6546     6513      -33     
+ Misses        944      853      -91     

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[maximlt]

A test would be much appreciated.

[maximlt]

In itself the change looks good to me. I'd be curious to know how geopandas' .plot behaves in the same situation.