Warn and render na_color when color column is all-NaN (#599)

Previously an all-NaN color column raised a misleading ValueError that
blamed instance-id alignment even when the join succeeded. Now:
- if dtypes differ (genuine alignment failure) the error is kept but
  rephrased to drop the incorrect "could not be aligned" language
- if dtypes match (all-NaN column) a warning is emitted and every
  element is rendered with na_color, consistent with partial-NaN behaviour

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: timtreis

src/spatialdata_plot/pl/utils.py

@@ -1104,12 +1104,18 @@ def _set_color_source_vec(
             element_label = _format_element_name(element_name)
             location = f"table '{table_name}'" if table_name is not None else "the element"
             dtype_hint = _build_alignment_dtype_hint(sdata, element, color_series, table_name)
-            raise ValueError(
-                f"Column '{value_to_plot}' for element '{element_label}' contains only missing values after aligning "
-                f"with {location}. This usually means the instance ids/indices could not be aligned or converted, so "
-                "colors cannot be determined. Please ensure the table annotates the element with matching instance ids."
-                f"{dtype_hint}"
+            if dtype_hint:
+                raise ValueError(
+                    f"Column '{value_to_plot}' for element '{element_label}' contains only missing values after "
+                    f"aligning with {location}. Please ensure the table annotates the element with matching instance "
+                    f"ids.{dtype_hint}"
+                )
+            logger.warning(
+                f"Column '{value_to_plot}' for element '{element_label}' contains only NaN values; "
+                "rendering with na_color."
             )
+            na_color_arr = np.full(len(color_series), na_color.get_hex_with_alpha())
+            return na_color_arr, na_color_arr, False
 
         kind, processed = _infer_color_data_kind(
             series=color_series,