fix(black-viewports): MPR viewports were turning black if their parent stack had it's properties reset (#5219)

extensions/cornerstone/src/services/ViewportService/CornerstoneViewportService.ts

@@ -345,6 +345,11 @@ class CornerstoneViewportService extends PubSubService implements IViewportServi
         delete properties?.voiRange;
         delete properties?.VOILUTFunction;
       }
+      if (properties?.colormap) {
+        if (properties.colormap?.opacity?.length === 0) {
+          delete properties.colormap.opacity;
+        }
+      }
       return properties;
     };