Warn when groups values don't match color categories (#455)

When `groups` contains values absent from the `color` column's categories,
emit a warning so users understand why the plot is empty. Also add a
defensive empty-input guard in `_get_collection_shape` to prevent a
`KeyError: 'geometry'` if an empty DataFrame ever reaches PatchCollection
construction.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: timtreis

src/spatialdata_plot/pl/render.py

@@ -302,6 +302,14 @@ def _render_shapes(
     # Only show non-matching elements if the user explicitly sets na_color.
     _na = render_params.cmap_params.na_color
     if groups is not None and values_are_categorical and (_na.default_color_set or _na.alpha == "00"):
+        assert color_source_vector is not None  # guaranteed by values_are_categorical
+        _groups_list = [groups] if isinstance(groups, str) else groups
+        _missing = set(_groups_list) - set(color_source_vector.categories)
+        if _missing:
+            logger.warning(
+                f"Groups {sorted(_missing)} not found in the values of '{col_for_color}'. "
+                "The `groups` parameter filters values of the `color` column."
+            )
         keep, color_source_vector, color_vector = _filter_groups_transparent_na(
             groups, color_source_vector, color_vector
         )
@@ -754,6 +762,13 @@ def _render_points(
     # Only show non-matching elements if the user explicitly sets na_color.
     _na = render_params.cmap_params.na_color
     if groups is not None and color_source_vector is not None and (_na.default_color_set or _na.alpha == "00"):
+        _groups_list = [groups] if isinstance(groups, str) else groups
+        _missing = set(_groups_list) - set(color_source_vector.categories)
+        if _missing:
+            logger.warning(
+                f"Groups {sorted(_missing)} not found in the values of '{col_for_color}'. "
+                "The `groups` parameter filters values of the `color` column."
+            )
         keep, color_source_vector, color_vector = _filter_groups_transparent_na(
             groups, color_source_vector, color_vector
         )

src/spatialdata_plot/pl/utils.py

@@ -553,6 +553,9 @@ def _create_patches(
         shapes_df, fill_c.tolist(), outline_c.tolist() if hasattr(outline_c, "tolist") else outline_c, s
     )
 
+    if patches.empty:
+        return PatchCollection([], **kwargs)
+
     return PatchCollection(
         patches["geometry"].values.tolist(),
         snap=False,