Add empty-layers guard to _additive_blend, clarify scalar_norm comment

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

Author: timtreis

src/spatialdata_plot/pl/basic.py

@@ -597,8 +597,9 @@ def render_images(
         n_steps = len(sdata.plotting_tree.keys())
 
         for element, param_values in params_dict.items():
-            # When norm is a list, per-channel norms are stored separately in ImageRenderParams.norms
-            # and _prepare_cmap_norm receives None so it creates a default (unused) norm.
+            # Per-channel norms are stored in ImageRenderParams.norms. In that case
+            # _prepare_cmap_norm gets None and creates a default norm used as fallback
+            # for the single-channel rendering path.
             scalar_norm = None if isinstance(norm, list) else norm
             cmap_params: list[CmapParams] | CmapParams
             if isinstance(cmap, list):

src/spatialdata_plot/pl/render.py

@@ -1201,6 +1201,8 @@ def _additive_blend(
     reasonable result by extracting each cmap's contribution relative to its
     own background.
     """
+    if not layers:
+        raise ValueError("Cannot blend an empty set of layers.")
     height, width = next(iter(layers.values())).shape
     zero_colors = np.array([cm(0.0)[:3] for cm in channel_cmaps])
     canvas = np.mean(zero_colors, axis=0)