Only warn about table fallback when multiple tables exist

When there is exactly one table, the choice is unambiguous — no need to
warn. The warning now only fires when there are multiple tables.

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

Author: timtreis

src/spatialdata_plot/pl/utils.py

@@ -1119,7 +1119,9 @@ def _set_color_source_vec(
             table_to_use = None
         else:
             table_keys = list(sdata.tables.keys())
-            if table_keys:
+            if len(table_keys) == 1:
+                table_to_use = table_keys[0]
+            elif len(table_keys) > 1:
                 table_to_use = table_keys[0]
                 logger.warning(f"No table name provided, using '{table_to_use}' as fallback for color mapping.")
             else: