fix broken color parsing as a result of fixing the metadata read

Author: r2dliu

src/datasource/zarr/ome.ts

@@ -111,7 +111,7 @@ function parseOmeroChannel(omeroChannel: unknown): SingleChannelMetadata {
   if (colorString && /^[0-9a-f]{6}$/i.test(colorString)) {
     colorString = `#${colorString}`;
   }
-  const color = parseRGBColorSpecification(colorString);
+  const color = colorString !== undefined ? parseRGBColorSpecification(colorString) : undefined;
   const inverted = getProp("inverted", verifyBoolean);
   const label = getProp("label", verifyString);