fix(ogc): keep the frame type when a result is empty

[thodson-usgs]

The bug

Every OGC getter returns a GeoDataFrame when its filter matches at least one row, and a plain DataFrame when it matches none.

locs = waterdata.get_monitoring_locations(state_name="Wisconsin")   # GeoDataFrame
locs = waterdata.get_monitoring_locations(state_name="Wisconsin",
                                          site_type_code="ZZ")      # DataFrame (!)

So .geometry, .to_crs(), .plot(), and pd.concat across a loop of per-site calls break only on the empty result — the case least likely to be covered by a caller's tests. It is also invisible from df.columns, because the geometry column is still listed.

Why

ogc/shaping.py::_empty_feature_frame exists precisely to prevent this; its docstring calls itself "the single home for this empty-page contract", because a later pd.concat([empty_page, geo_page]) would otherwise downgrade the result and strip geometry/CRS.

But _deal_with_empty runs last, on every getter, 160 lines away in the same file — and built a fresh pd.DataFrame(columns=properties), discarding the guarantee it had just been handed.

The fix

Reindex the frame we were given instead of constructing a new one. That applies the schema's columns while keeping the concrete class — and the geometry dtype with it:

GeoDataFrame().reindex(columns=["monitoring_location_id", "geometry"])
# -> GeoDataFrame, geometry dtype preserved, empty

The plain-pandas path is unchanged: pd.DataFrame().reindex(columns=[...]) is still a pd.DataFrame.

Testing

• New regression test uses a DataFrame subclass rather than real geopandas, so the invariant is checked even where geopandas is absent (it is not in the [test] extra, only [nldi] and [doc]). Verified to fail against the old line and pass against the new one.
• 788 passed; mypy --strict, ruff, xenon, complexipy, lint-imports all pass.

Found by the scan in #372, split out because it changes user-visible behavior. Independent of #372 — branches off main.

🤖 Generated with Claude Code

https://claude.ai/code/session_01BTaSm7HmVb94RSJiKW4WAS