Support `from_duckdb` with no property columns

[kylebarron]

Discussed in #619

^{Originally posted by aborruso September 2, 2024}
Hi,
if in my below code I remove the id_bene field and thus only one geom field is present

sql = """
CREATE TABLE rides AS
SELECT
  id_bene,ST_Point(longitudine, latitudine) as geom
FROM
  parquet_scan('../data/beni_immobili_pubblici_geo.parquet');
"""

con.execute(sql)

con.table("rides")

layer = HeatmapLayer.from_duckdb(con.table("rides"), con)

m = Map(layer)
m

I have these errors

AttributeError                            Traceback (most recent call last)
Cell In[11], line 24
     20 con.execute(sql)
     22 con.table("rides")
---> 24 layer = HeatmapLayer.from_duckdb(con.table("rides"), con)
     26 m = Map(layer)
     27 m

File ~/.local/lib/python3.11/site-packages/lonboard/_layer.py:1689, in HeatmapLayer.from_duckdb(cls, sql, con, crs, **kwargs)
   1680 @classmethod
   1681 def from_duckdb(
   1682     cls,
   (...)
   1687     **kwargs: Unpack[HeatmapLayerKwargs],
   1688 ) -> Self:
-> 1689     return super().from_duckdb(sql=sql, con=con, crs=crs, **kwargs)

File ~/.local/lib/python3.11/site-packages/lonboard/_layer.py:372, in BaseArrowLayer.from_duckdb(cls, sql, con, crs, **kwargs)
    370     table = _from_duckdb(rel, con=con, crs=crs)
    371 else:
--> 372     table = _from_duckdb(sql, con=con, crs=crs)
    374 return cls(table=table, **kwargs)

File ~/.local/lib/python3.11/site-packages/lonboard/_geoarrow/_duckdb.py:57, in from_duckdb(rel, con, crs)
...
     95     geom_col_name = rel.columns[geom_col_idx]
     97     # A poor-man's string interpolation check
     98     # We can't pass in SQL-templated strings for the column name

AttributeError: 'NoneType' object has no attribute 'arrow'

Why is it not possible to have a heatmap from only a geom field?

If it were possible, I would enable it.

Thank you for this great product